96 PARTS OF BIVALVE SHELLS. 



when the aperture is placed downwards, the nature of the spire 

 runs upwards from the right hand to the left {fig. 25). 



Chambers are the cavities, divided by partitions at regular or 

 irregular intervals, as in the Nautilus {fig. 15, page 30). 



Umbilicus is a circular perforation in the base of the lower 

 whorl or body. {See page 38). 



Sub-vmbilicated shells^ are those which have the umbilicus 

 covered, in a greater or less degree, by a thin process ; which, 

 in some, almost entirely closes the aperture or mouth. This 

 character is most commonly to be met with among species of 

 Buccinum and Murex. 



Umbilical fissure, is a groove extending from the umbilicus 

 {fig. 116). 



Shells which have no umbilicus are termed imperforate. 



Siphmiculus (little syphon) is that small round perforation 

 which forms a communication between the chambers in the 

 Nautilus, and penetrates through the whole spire of the shell 

 {figs. 15 and 16). 



Varices are transverse ribs which cross the whorls of shells 

 in some species of Buccinum, Murex, &c. Varices are formed 

 by the periodical growth of the shells, these being the margin of 

 the outer lip, to which the animal has attached its periodical 

 enlargements {fig. 46). In some species they have more the 

 form of sutures than ribs ; this is owing to the margin of the 

 outer lip being but slightly developed. 



Ribs are those longitudinal and transverse protuberances 

 which are in many of the univalve shells {fig. 46). 



Teeth of univalves, according to Colonel Montagu, are not 

 properly tooih-shaped protuberances, but are fine white laminae 

 or ridges, running spirally backwards, in a direction parallel to 

 each other ; those on the exterior lip may, in most instances, be 

 traced through the outside of the shell, and are nearly alike in 

 length {figs. 24, 25 and 42). 



Epidermis is a skin, or cuticle, covering the exterior surface 

 of many shells, destined by nature to protect their surface from 

 being injured. 



True bivalve shells are peculiar to the acephalous mollusca; 

 and their presence is constant, although they are in a few 

 instances too small to cover the whole body, and in the ship- 

 borers {Teredo) exist only as small instruments, limited to the 

 function of excavating the burrows inhabited by these moUusks. 

 But all the species, in which the bivalve shell is inadequate to 

 .the protection of the whole body, derive extrinsic defence by 

 burrowing in sand, or stone, or wood ; and they also commonly 

 line their burrows with a layer of smooth and compact caU 



