Introdnctio7i to Animal Morphology. 255 



Chonetidae (PalEeozoic) has a tooth-bearing hinge and tubular 

 spines. Strophomenidae (Palaeozoic, Leptaena, also Liassic) 

 is similar, but with no spines. Productid^ has no teeth in 

 the hinge, only a straight line, and the dorsal valve often 

 concave. 



CHAPTER XXXV. 

 DIVISION 2. — OTOCARDIA [Haeckel). 



Molluscs having hearts consisting of one or two 

 auricles and one ventricle. The integument is dif- 

 ferentiated dorsally into a mantle, ventrally into a 

 foot. The nervous system consists of a pharyngeal 

 ring with, usually, epi- and hypo-pharyngeal ganglia, 

 and one or more visceral ganglia united to the epi- 

 pharyngeal by commissures. Three classes are in- 

 cluded herein : — 



I . Lamellibranchiata [Blatnvillef — aquatic, mostly 

 marine, fixed or free; with a large mantle usually 

 covering the whole body, and consisting of two, more 

 or less, symmetrical lateral lobes, continuous from 

 before backwards, and overhanging the mouth, at- 

 tached together medio-dorsally, and secreting on its 

 outer surface a shell of two lateral valves. There is 

 no specialized head-segment with centralized sense- 

 organs. The shell is often equivalve (except at the 

 hinge). The valves are rarely equilateral,! and are 

 united medio-dorsally by a longitudinal, movable 



* Acephala, Cuvier ; Bivalvia, Linnaeus. 

 t As in Pectunculus. 



