TYPE MOLLVSCA. 



309 



from the other Monotocardia, while others are extensively 

 modified. The genus Atalanta possesses a large transparent 

 shell within which the animal can be completely retracted. 

 The foot is no longer adapted for creeping, but is dilFerenti- 

 ated into a laterally flattened keel-like pro- and mesopodium 

 which bears a sucker on its posterior surface, and a metapo- 

 dium provided with an operculum. In Carinaria (Fig. 138) 



Fig. 138. — Stkuctuke of Garinaria mediterranea (after Owen). 



ao = aorta. 



b = buccal mass. 

 eg — cerebral ganglion. 

 ct = ctenidium. 

 h = heart. 



i = intestine. 



I = liver. 

 mp = mesopodium. 



= eye. 

 p = penis. 

 peg = pedal ganglion, 

 s = salivary gland. 

 su = sucker. 

 te = testis. 

 •vd = vas deferens. 

 vg = visceral ganglion. 



m = seminal vesicle. 



the visceral hump is reduced to a comparatively small mass 

 upon the dorsal surface of the elongated body and is enclosed 

 in a transparent shell shaped like a liberty-cap. The pro- and 

 mesopodium have the form of a plate hanging down from 

 about the middle of the under-surface of the body, and the 

 metapodium is directed backwards, forming in reality the 

 posterior portion of the body. The same relationships of the 

 foot are found in Pterotrachea, which presents the extreme of 

 modification found in this group ; in this form the visceral 

 hump is still more reduced than in Carinaria, forming only a 

 small oval mass imbedded in the dorsal surface of the body 

 and being destitute of any shell. Considering these two forms, 



