120 AMERICAN JOURNAL 



ON THE PTEROCER-ffi OF LAMARCK, AND THEIR MUTUAL 



RELATIONS. 



BY THEODORE GILL, M, D. 



From the earliest times of scientific zoology, there has heen 

 almost universally recognized as a natural group,* and with 

 its proper affinities as since demonstrated, that group of living 

 Strombidfie provided with a digitated expanded lip, with which 

 the Lamarckian name of Ptcrocera has been most generally 

 associated. Whether the mutual relations of the components of 

 that group have been equally well appreciated will be the sub- 

 ject for present inquiry. 



§ 1. Differential characters. 

 1. Primary sjnnes and their homologies. 



Accepting the current views, ten recent species of Pterocera, 

 ■as defined by Lamarck, are distinguishable, and these differ 

 In the number of alar digitations, the direction of the digitations, 

 their distribution, and their relative size, the form of the aper- 

 ture, as well as the presence or absence of rugosities on the 

 lip, and other trivial characters. 



As to the number of digitations, the difference would at first 

 appear to be very great, but more extended study demonstrates 

 that it is more apparent than real, and that, in the case of 

 species with an enlarged number of digitations, a distinction 

 should be made between primary and secondary or intercalary 

 ones. 



Eliminating for the present such secondary or intercalary 

 digitations, the species of the group are distributable into two 

 minor groups ; onedistinguislied by the major development of six 

 primary digitations, (exclusive of the canal), and the other by 



* To it, however, bave been referred species which are not closely re- 

 lated to the species of the natural group, and which liave only an analo- 

 gical resemblance to such. 



