204 EOCENE MOLLUSCA. 



When Lamarck first established the present genus, he at the same time separated 

 the species whose shells terminate in a short canal, under the name Clavatula, a 

 genus which, as defined by the author, rested wholly on characters taken from the 

 shell, and which was afterwards withdrawn by him as not tenable. In this suppression 

 subsequent writers have concurred until recently, when, a better knowledge of the 

 animal having been acquired, characters have been pointed out which are generally 

 considered as sufficient to justify the separation, and the genus has accordingly been 

 adopted, and has been re-defined by Dr. Gray. The most prominent of these charac- 

 ters are the shape of the foot, which is large and ovular, and the condition of the 

 operculum, the nucleus of which, instead of being apical as in the true Pleurotoma, is 

 placed nearly at the middle of the right margin. The shell also, it must be observed, 

 presents an emargination near the anterior extremity of the outer lip, similar to that 

 found among the Strombida, and a thick callosity at the hind part of the columella near 

 the suture, characters which are not found in the present genus. 



The genus Pleurotoma, as originally defined, comprised a large number of species, 



and the list of synonyms shows how strongly the necessity for some subdivision has 



been felt. Some of the proposed divisions, however, so far as the present imperfect 



knowledge of the animals will enable the student to estimate their value, appear 



to rest on conchological distinctions, and as yet are not generally accepted. In others, 



however, the animals exhibit distinct characters, which, taken in conjunction with 



modifications of the shells, are received as of sufficient generic value. Thus in 



Bela (Leach), the eyes are placed on the upper part of the tentacles, which are 



approximate, and the outer lip of the shell is simple, or with a slight sinus confluent 



with the suture ; and in Mangelia (Leach; Defrancia, Millet; Maphitoma, Bellardi), 



the eyes are sub-pedicelled, the foot is short, tapering behind, and enlarged in front at 



each corner into a hook-shaped projection, and the dentition, according to Messrs. 



Forbes and Hanley, presents a simple rachidian tooth in addition to the two lateral 



subulate teeth of the present genus ; the animal, also, is without an operculum, 



and the outer lip of the shell has a sinus resembling that found in the notched 



species of Bela.* Again, in Perrona (Schum. ; Tomella, Swains.), the nucleus of 



the operculum is placed near the middle of the right side, as in Clavatula, of which 



genus, in fact, it appears to be only a smooth form ; and again, in Drillia (Gray), 



according to Messrs. Adams, the eyes are placed near the lips of the tentacles, which 



are very slender and approximated, and the outer lip presents a small sinus in front, 



like the one in Clavatula already noticed. A more extended knowledge of the animals 



may hereafter support others of the proposed genera ; but, in the mean time, they can 



be received only as subdivisions for facilitating the arrangement of the very numerous 



species which crowd the present genus. 



* Messrs. Forbes and Hanley have united Mangelia and Bela, using the latter name to distinguish the 

 operculated species from those without an operculum, which, with those authors, form the true Muityelice. 



