Chapter XIX: Section II: CHITONES IRREGULARES.--Posterior valve abnormal, or

with a sinus behind._

E. _Schizoidea._--Posterior valve fissured; _Lorica_,
_Schizochiton_.

F. _Placiphoroidea._--Posterior valve unslit, internally
ridged, umbo nearly terminal; _Enoplochiton_, _Ornithochiton_,
_Plaxiphora_.

G. _Mopaloidea._--Posterior valve with posterior sinus and one
slit on each side; _Mopalia_, _Katherina_, _Acanthochiton_,
_Notoplax_.

H. _Cryptoidea._--With double sutural laminae; _Cryptoconchus_,
_Amicula_, _Cryptochiton_.

I. _Chitonelloidea._--Posterior valve funnel shaped; laminae
thrown forward; _Chitonellus_, _Choneplax_.

=Sub-order 2.= =Aplacophora.=--Animal vermiform, foot absent, or a mere groove, cuticle more or less covered with spicules.

According to Marion, one of the principal authorities on the group, the Aplacophora are perhaps Amphineura whose development has been arrested at an early stage, their worm-like exterior being due to adaptation to surroundings. They have hitherto been found chiefly in the N. Atlantic and Mediterranean, generally at considerable depths, and often associated with certain polyps in a way which suggests a kind of commensalism.

FAM. 1. _Neomeniidae._--Foot a narrow groove, intestinal tube without differentiated liver, kidneys with common exterior orifice, sexes united, ctenidia present or absent. Genera: _Neomenia_ (Fig. 266), _Paramenia_, _Proneomenia_, _Ismenia_, _Lepidomenia_, _Dondersia_.

FAM. 2. _Chaetodermatidae._--Body cylindrical, no ventral groove, liver a single sac, kidneys with separate orifices into the branchial cloaca, two bipectinate ctenidia. Single genus, _Chaetoderma_ (Fig. 267).

=Order II. Prosobranchiata=

Visceral loop twisted into a figure of 8 (streptoneurous), right half supra-intestinal, left half infra-intestinal; heart usually in front of the branchia (ctenidium), which is generally single; head with a single pair of tentacles; animal dioecious, usually marine, more or less contained within a shell, operculum generally present. Cambrian to present time.

=Sub-order 1.= =Diotocardia.=--Heart with two auricles (except in the _Docoglossa_ and _Helicinidae_), branchiae bipectinate, front end free; two kidneys, the genital gland opening into the right (except in Neritidae); nervous system not concentrated; no proboscis or siphon, penis usually absent.

(_a_) DOCOGLOSSA (p. 227).--Heart with a single auricle, ventricle not traversed by the rectum, visceral sac not spiral, shell widely conical, non-spiral, no operculum; radula very long, with few hooked teeth in each row.

FAM. 1. _Acmaeidae._--Left ctenidium alone occurring, free on a long stalk. Cretaceous----. Principal genera: _Pectinodonta_, front part of head much produced, radula 0 (1. 0. 1.) 0; _Acmaea_ (= _Tectura_), with sub-genera _Collisella_ and _Collisellina_, no accessory branchial ring, shell closely resembling that of _Patella_, but generally with a distinct internal border; _Scurria_, accessory branchial ring on the mantle.

FAM. 2. _Lepetidae._--No ctenidia or accessory branchiae, animal generally blind. Pliocene----. Principal genera: _Lepeta_; _Propilidium_, apex with internal septum; _Lepetella_.

FAM. 3. _Patellidae._--No ctenidia, the osphradial patch at the base of each alone surviving, a circlet of secondary branchiae between the mantle and sides of the foot. Ordovician----. (i.) _Patellinae._--Three lateral teeth on each side, two of them anterior. Principal genera: _Patella_, branchial circlet complete; chief sections _Patella_ proper, _Scutellastra_, _Ancistromesus_ (_A. mexicana_ Brod., measures 8–14 in. long); _Helcion_, branchial circlet interrupted in front; _Tryblidium_ (Ordovician).--(ii.) _Nacellinae._--Two developed laterals on each side, one anterior. Genera: _Nacella_, branchial circlet complete; _Helcioniscus_, branchial circlet interrupted in front.

(_b_) RHIPIDOGLOSSA (p. 225).--Ventricle of the heart traversed by the rectum (except in Helicinidae), one or two ctenidia; jaw in two pieces, radula long, marginals multiplied, rows curved.

Of all the Gasteropoda, this section of the Diotocardia approach nearest to the Pelecypoda, particularly in the least specialised forms. The auricle, the branchiae, and the kidneys are in many cases paired, and more or less symmetrical. The ventricle is generally traversed by the rectum, there is a long labial commissure between the cerebral ganglia, special copulative organs are usually absent, while the shell is often nacreous, like those of Pelecypoda of a primitive type.