184 AMERICAN JOURNAL 



O. ehurnea. Ampliiperas ovum, however, is one of the most 

 aberrant forms of the family to which it has given its name, 

 under the patronymic form, and Radius voJva is one of the most 

 typical, — that is, it exhibits the plan of growth in its greatest 

 simplicity. 



In all the Cyprseidae the shell exhibits at the posterior end the 

 evidences of a spire, although sometimes almost or entirely con- 

 cealed from view in the adult, and even sunk in an umbilical 

 depression, produced by the accumulation of callous around it. 

 In the young, however, the spire is very distinct, though it may 

 be either depressed or eb'vated, and the whorls are wound 

 around an axis as in all spiral shells, the modified form of the 

 adult shell being: the result of the great successive increase of 

 the whorls, the closeness of the winding near the sutures, the 

 inflection of the labrum and the expansion of a callous deposit 

 over the lips. 



On the other hand, the Amphiperasid?e have not even spiral 

 shells, the testaceous envelope being simply a shell loosely rolled 

 on itself, and more or loss attenuated and twisted at the extre- 

 mities, and of course a spire is never seen either in young* or 

 old, and the only feature of resemblance in the shell is the infla- 

 tion of the testaceous roll, combined with the proportionately 

 little attenuation of the extremities, the inflection of the outer 

 lip, and that extension at the extremities which is the co-ordi- 

 nate of the peculiar plan of growth. The resemblance in form 

 of any species of the Amphiperasidse to Cyprseidse is therefore 

 simply analogical, and the elements for an exact comparison do 

 not exist. The resemblance between Cyproeid?e and certain 

 Cassidi(loe and Marginellidse is homological, and based on simi- 

 lar modific:itions of growth; but in Amphiperasidi« the reverse 

 is decidedly the case. And it may still further be remarked 

 that, while the shells of Cyprgeidae, Planorbinae, Terebridge and 

 HaliotidjB are comparable :imong themselves, because they all 

 exliibit modifications of a spiral, the comparison cannot be ex- 

 tended to the Amphiperasidaj. But, at the same time, it may 

 be added that the morphological problem of the conversion of 

 the spiral into the roll is not a diflicult one, and various forms, 

 otherwise closely allied, are found among the Tectibranchiates, 

 distinguished from each other by the development of a spiral or 

 simply rolled testaceous envelope. Among the Pectinibranchiates, 

 however, the diff'erence is the co-ordinate of other and more im- 

 portant ones. An illustration may serve best to render the con- 

 trast obvious. A circle and an ellipse are two fundamentally 



* The embryonic coudition is not referred to, and is unknown. 



