Chapter XXXV: Palaeozoic Gymnospermous Seeds (3)

The cast represented in fig. 506, A, from the Middle Coal Measures of Yorkshire is 3·5 cm. long and has 18 regular longitudinal ribs: it is radially symmetrical and does not conform in this respect to Arber’s definition of _Platyspermum_. In many cases, _e.g._ the flattened seed figured by Kidston as _Rhabdocarpus multistriatus_, it is impossible to determine the symmetry of the seed. The old generic name _Carpolites_, _Carpolithes_ or _Carpolithus_ has been used in a very wide sense and does not connote any well-defined features; it should be reserved, in the form _Carpolithus_ as used by Linnaeus, for seeds that cannot be assigned to a systematic position or which do not exhibit any distinctive characters worthy of emphasis by the institution of a special name. The generic term _Holcospermum_, recently proposed by Nathorst[1056], is a suitable name for this type of seed; as defined by the author, it includes seeds that may be radiospermic or platyspermic. Specimens of the type-species, _H. dubium_[1057],from the Lower Carboniferous beds of Spitzbergen, agree closely in size and shape with some examples of _Rhynchogonium_ and _Boroviczia_, but they are distinguished by prominent ribs extending the whole length of the cast. In some seeds similar to _H. sulcatum_ the ribs form sharp ridges, but the difference between rounded and sharp ridges is often determined by the method of preservation: a specimen deprived of its outer flesh would form a cast more sharply ribbed than a seed in which the sarcotesta had been moulded on to the ribbed sclerotesta. On the other hand some ribs are formed by blunt sclerotestal folds as in _Colpospermum_: both types of ribbing are included in the genus _Holcospermum_.

ii. _Other Genera._

=Malacotesta.= Williamson.

This generic name was instituted by Williamson[1058] for a petrified seed from the Coal Measures of Lancashire which he named _Malacotesta oblonga_: the type-specimen is a small seed 6–7 mm. long characterised by a thick fleshy integument. The structure is however insufficiently known to admit of a satisfactory comparison of the imperfectly preserved specimen figured by Williamson with other types.

=Thysanotesta.= Nathorst.

Nathorst[1059] founded this genus on a single seed from the Lower Carboniferous plant-beds of Spitzbergen which exhibits features sufficiently distinctive to justify its recognition as the type of a separate genus.

_Thysanotesta sagittula_ Nathorst. The type-specimen is a long and narrow seed, 20 mm. long, ovate below and prolonged apically into a slender beak, 8 mm. in length, which probably represents a micropylar tube; the beak bears numerous stiff hairs (fig. 506, F). The seed closely resembles a carpel of _Erodium_ without the horizontal part of the awn. There is no evidence as to the nature of the parent-plant but the species affords an interesting example of a Palaeozoic seed apparently adapted for wind-dispersal, or possibly the bristles may have served the same purpose as in the fruits of _Erodium_. Attention is called elsewhere[1060] to the frequent resemblance of Palaeozoic seeds such as _Thysanotesta_ to fruits of recent flowering plants.

=Carpolithus.= Linnaeus.

Nathorst[1061] has pointed out that this generic name was employed by Linnaeus for fossil fruits: in the form _Carpolites_ (Sternberg) or _Carpolithes_[1062] it has been widely used and differently defined by authors, usually in a comprehensive sense including fossil seeds from both Palaeozoic and Mesozoic strata which cannot be assigned to a definite position in the plant-kingdom. It is desirable to adopt some designation for seeds from strata of different ages which do not exhibit any features sufficiently distinctive to justify the creation of a special genus. By employing such a name as _Carpolithus_, without attempting to define its characters within prescribed limits, for casts or impressions of seeds which are not distinguished by any striking characters and cannot be allocated to any particular section of seed-bearing plants the unnecessary multiplication of generic titles is avoided: when any additional data are obtained differentiating particular types from other forms of _Carpolithus_ the provisional term should be superseded by some distinctive generic name. Among seeds from the Coal Measures there are several examples of both large and small types without any regular ribs or lacking such features as serve to distinguish the genera already described, which are appropriately included in _Carpolithus_. The two species _Carpolithus Wildii_ Kidst.[1063] and _C. bivalvis_ Goepp.[1064] are two examples of seeds from Upper Carboniferous rocks which it is desirable to refer to this comprehensive genus pending further discoveries as to their morphological features. The Jurassic species _C. conicus_ Lind. and Hutt.[1065] is another type which it has been customary to include in _Carpolithus_.

(_Microspermum._ Arber.)

The generic name _Microspermum_ has recently been proposed by Arber[1066] for some Westphalian seeds described by Carpentier[1067] from the North of France as _Carpolithes? samaroides_ and for similar fossils from the Middle Coal Measures of Nottingham. The supposed seeds are small, ovate or pyriform bodies, 5–12 mm. long and 2·5–6 mm. broad, with one extremity broadly rounded and the other acute: one side is keeled, the other grooved, and a large foramen occurs near the broader end and on the grooved surface. Arber emphasises the fact that the specimens are symmetrical in one plane only. As the author of the genus points out the morphological nature of the fossils is not thoroughly established, nor is the parent-plant known. In view of the ill-defined characters of the specimens so far discovered it is hardly desirable to institute a new generic name implying their seed-nature; moreover _Microspermum_ has previously been employed for a genus of Compositae. An examination of specimens leads me to doubt their seed-nature and it would seem more likely that they are foliar organs, possibly bracts which originally bore seeds or sporangia.

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An examination of some of the specimens of supposed Permian seeds for which Geinitz[1068] proposed the generic name _Guilelmites_ convinced me that they are inorganic structures, probably nodules in shale smoothed and rounded by slickensiding.