370 Mr. Robert Garner's Malacological Notes. 



side of the body appear to have become atropliied ; and here 

 we are to suppose tliat the shell, as seen from above, is homo- 

 logically univalve, with a strong tendency to become spiral, 

 in fact so strongly centrifugal in Calyptrcea as to have carried 

 the heart into an unusual position, and almost to have disin- 

 tegrated the shell ; whilst in the second case the whole con- 

 formation of heart, branchiffi, foot, muscles, and organs of 

 generation agrees so certainly with that of bivalves, that we 

 must have in one of these molluscous shells the two shells of 

 the bivalve in union. In Patella the branchise continue situ- 

 ated at the sides of the foot, though the heart is to the left, 

 whilst the rectum opens on the right side of the neck ; in Emar- 

 ginula the pedal fringe is but rudimentary, and the true bran- 

 chiae are seen above the neck — a disposition (in the last respect) 

 normal in most univalves ; but in this case the ventricle is 

 perforated by the rectum. Calyptrcea and, more markedly, 

 H{pj)onyXj being often fixed on very irregular surfaces, have 

 the power of levelling for themselves a foundation by the secre- 

 tion of a shelly pedestal. It is puzzling to say how this is 

 formed, or with what part in other mollusks it is homologous. 

 It must either be the representative of the byssus, or, more pro- 

 bably, what appears to be the foot-disk is in reality an opercular 

 surface secreting the support, which is consequently marked 

 with muscular impressions. There are lappets in front of the 

 disk in these genera ; correspondingly the anterior dorsal disk 

 in Sigaretus is not homologous with the similar anterior dorsal 

 surface of Bullcea ; the situation of the mouth shows this. 



The form of the univalve shell, due primarily to original 

 plan, or, in other words, to an adaptation of ways to ends, 

 depends, secondarily and immediately, on the form of the 

 mantle-opening, on its inclination from the action of the foot, 

 and on the relative activity of its circumference with respect 

 to the secretion of the shelly matter. An increasing tube 

 would be prolonged in a straight line or nearly so, if the mantle 

 at the circumference were equally active and not unequally 

 inclined, as, for example, in Dentalium, which is in all respects 

 near to the Gastropods lately dwelt upon, Fissurella^ &c. ; but, 

 on the contrary, it would be more curved if one side were more 

 active or more extensile. VermetuSj Siliquaria, and Magilus 

 have an unequal deposit at first, and are then spiral ; after- 

 wards the shells become disjoined and approach the straight 

 line. In Patella the whole circumference of the mantle 

 secretes and grows almost equally and rapidly, hence the de- 

 pressed conical form ; whilst Capulus is at first less rapidly 

 expanded, with the axis of increase changing somewhat, 

 whence it attains its peculiar form and is named " Hungarian 



