22 AMERICAN JOURNAL 



division and which are as completely as the Helicinge deprived 

 of a median furrow of the foot. Here again there is no reason 

 for a separation. 



As to the reasons for placing them together, on the other hand, 

 they are numerous and evident. The Cyclostomacea and the 

 Helicinacea are equally pulmoniferous, and have equally the 

 eyes placed at the external base of the tentaculge. These ten- 

 taculge have exactly the same elongated and pointed form in the 

 Cycloplioriis and Helicma. The animals of these two groups 

 have the same habits, the same way of living. Lastly, and for 

 us it is conclusive, there exists a certain number of generic forms, 

 placed, so to saiy, a-straddle of these groups, borrowing their 

 characters of one and the other at the same time, and conse- 

 quently uniting them, despite of all divisions, however ingenious 

 they may be. 



The curious genus Bourciera, placed by Mr. Pfeiffer among 

 the Cyclostomacea, has an operculum with a distinctly spiral 

 structure, a lingual dentition as in the Helicinge* an heliciniform 

 shell, but it is without lustre, deprived of collumellar callosity, 

 and presenting, in the umbilical part, some resemblance with cer- 

 tain Cyclostomas. Of tliese three characters, the first makes of 

 the Bourciera a Cyclostomacea, the second a Helicinacea, and 

 the third an intermediate form, approaching more to the Helici- 

 nge than to the Cyclostomce. 



The genus Georissa, recentlyf established by Mr. W. T. Blan- 

 ford, comprises animals which, although they have a shell having 

 the appearance and the characters of JTi/d7'ocena, differ by hav- 

 ing a non-spiral operculum, excentrically striated, semi-oval, and 

 approacliing therefore much more to the Helicinacea than to the 

 Cyclostomiicea. Before Mr. Blanford had described this singular 

 operculum, the species of this small Indian group were considered 

 by Mr. Benson, Avho had described them, and by Mr. Pfeiffer, 

 in the first Supplement to his "Monograph of the Pneumono- 

 poma " to be Hydrocena (H. pixis, JI. ilJex and H. twTita, 

 Benson), and this for the simple reason that, from the appear- 

 ance of the shell, it was impossible to distinguish them from the 

 other species of that genus in any other way than specifically. 

 The knowledge of the operculum has decided Mr. Pfeiffer, who, 

 as it is well known, attaches a great deal of systematic import- 

 ance to that character, to put the Georissa in another family in 

 his second Supplement, but it has taken nothing from the 

 conchological characters which make of this small genus a new 

 proof of the intimate connection of the Helicinacea with the 

 Cyclostomacea. 



* Troschel, 1. c, I, p. 246, pi. XX, fig. 14. 



t W. T. Blanford, Ann. and Mag. Nat. Hist., 1864. 



