Introduction to Animal Morphology. 255 



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

 spines. Strophomenidae (Palaeozoic, Leptaena, also Liassic) 

 is similar, but with no spines. Productidae has no teeth in 

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

 concave. 



CHAPTER XXXV. 



DIVISION 2. OTOCARDIA (Haeckcl}. 



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 (Blainmllcf 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 



ecialized head-segment with central i/rd sense- 



Thf shell is often equivalve (except at the 



hinge). The valves are ran-lv equilateral,! and are 



'-d^rsally by a longitudinal, inovaMe 



ctunculus. 



