270 MOLLUSCA. 



CLAVAGKLLA, Lamarck, 



One of the valves is clasped by the tube, leaving the other, however, free. A 

 single living species is found in the Madrepores of the Sicilian seas, which has 

 been described by M. Audouin. 



Some naturalists think we should also place in this family the 



ASPERGILLUM, Lamarck, 



The shell of which is formed of an elongated conical tube, closed at its widest 

 extremity by a disk perforated with numerous small tubular holes ; the little 

 tubes of the outer range being longest, form a kind of corolla round it. The 

 reason for aproximating them to the Acephala with tubes is found in the fact 

 that there is a double projection on one part of the cone which really resembles 

 the two valves of the Acephala. The affinity between these little tubes and 

 those which envelope the tentacula of certain Terebella, formerly caused this 

 animal to be refe.red to the Annulata. 



The species most known, Asper. Javanum, is seven or eight inches in length. 



ORDER II. 



ACEPHALA NUDA. 



THE naked Acephala are not numerous, and are sufficiently removed from 

 the ordinary Acephala to form a distinct class, were such a division considered 

 requisite. Their branchiae assume various forms, but are never divided into 

 four leaflets ; the shell is replaced by a cartilaginous substance which is some- 

 times so thin that it is as flexible as a membrane. We divide them into two 

 families. 



FAMILY I. 



SEGREGATA. 



THIS family comprises those genera in which the individuals that compose 

 them are insulated and without any mutual organic connection, although 

 frequently associating together. In the 



BIPRORA, Brugtiieres, THAMA, Brown, SAI.PA and DAGYSA, GmeHn, 



The mantle atid its cartilaginous envelope are oval or cylindrical, and open at 

 the two extremities. Muscular bands embrace the mantle and contract the 

 body. The animal moves by taking in water at the posterior aperture, and 

 forcing it out through that near the mouth, so that it is always propelled back- 



