184 AMERICAN JOURNAL 



C. ehurnea. Amphiperas ovum, however, is one of tlie most 

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

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

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

 simplicity. 



In all the Cyprseidje 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, ihuugh it may 

 be either depressed or elevnted, and the whorls are Avound 

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

 adult shell beincr the result of the great successive increase of 

 the whorls, the closeness of the Avinding near the sutures, the 

 inflection of the labrum aad the expansion of a callous deposit 

 over the lips. 



On the other .hand, the Amphiperasidre have not even spiral 

 shells, the testaceous envelope being simply a shell loosely rolled 

 on itself, and more or less attejmated 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 exten&ion 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 Cyprseidaa is therefore 

 simply analogical, and the elements for an exact comparison do 

 not exist. The resemblance between Cypn^idie and certain 

 Cassididje and Marginellidfe is homological, and based on simi- 

 lar modifications of growth; but in Amphiperasidie the reverse 

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

 that, while the shells of Cyprgeidse, Planorbinae, TerebridfB and 

 IlaliotidjB are comparable among themselves, because they all 

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

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

 be added that the morphologic;! 1 problem of the conversion of 

 the spiral into the roll is not a difficult 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 difi'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 condition is not referred to, and is unknown. 



