CONCHOLOGY. 



Canal, or gutter, an elongation of the 

 aperture of the shell descending in a 

 groove or gutter-like process. Some 

 kinds of rostrated shells have the canal 

 remarkably conspicuous, forming a sinus 

 from the aperture throughout the whole 

 length of the beak. 



Umbilicus, is the opening or perfora- 

 tion in the lower part of the body, or 

 first whorl of many spiral univalves, and 

 is very conspicuous in a number 'of the 

 trochus and nerita genus in particular. 

 This umbilical perforation runs in a 

 straight line from the base to the sum- 

 mit of the shell, forming throughout a 

 spiral groove or gutter, which is wide at 

 the entrance, and tapers gradually to- 

 wards the apex. In the Linnxan nerita 

 canrena, the structure of the umbilicus 

 is well displayed, but is still more obvious 

 in the staircase shell, trochus perspecti- 

 vus. This opening occurs in many shells 

 at the base of the pillar. 



Operculum, is a testaceous or cartilagi- 

 nous appendage, peculiar in a consider- 

 able degree to the univalve tribe of shells, 

 and those only of the spiral or turbinated 

 kinds. This appendage is not connected 

 with the shell, but the animal, and serves 

 like a lid or little door, to protect or close 

 up the aperture of the chamber, when 

 the creature retires within its habitation. 

 Shells of this kind are distinguished by 

 the name of cochleae operculatze, by some 

 of the elder conchologists. The oper- 

 cula are often small in comparison to the 

 size of the shell to which they belong; 

 their form varies in different species ; 

 and their substance in some of a horny 

 texture, and in others testaceous, or ap- 

 proaching the nature of stone. Their 

 figure in common is either perfectly 

 round, elliptical, oval, or elongated, and 

 sometimes wrought with spiral work, or 

 concentric lines. 



Epidermis, is a kind of skin or coating 

 with which the exterior surfaces of many 

 shells, both of the univalve and bivalve 

 tribe, are covered. It is considered as a 

 sort of periosteum or membrane, design- 

 ed by nature to defend the shell from ac- 

 cidents and aid their growth, and to pre- 

 vent other testaceous or marine animals 

 from fixing their habitations on these 

 shells, as they do upon most bodies in 

 the sea, where there is no power of re- 

 sistance. The epidermis is a genuine co- 

 vering formed by the animal itself, pecu- 

 liar to some kinds, and as constantly 

 never observed on others. There is no 

 doubt but the animal, to which this sort 

 of covering is peculiar, possesses a pro- 

 per apparatus for its construction. The 



structure of this epidermis, it should b 

 added, is very distinct in different shells, 

 consisting in some of a very thin pellucid 

 film, and in others laminated, pilous, vel- 

 vetty, fibrous, or rugged. Few shells, 

 having a rugose surface, are destitute of 

 this external covering or epidermis. 



/Jivalves,.or shells of two valves united 

 by means of a cartilage, hinge, connection 

 of the teeth, or other process. In order to 

 constitute a bivalve shell, it is only requi- 

 site that it be furnished with two connect- 

 ed valves, without regard to their resem- 

 blance in form or dimensions. Some of 

 the bivalves have both valves formed alike; 

 in others they differ only in a slight de- 

 gree, and again in others they are altoge- 

 ther dissimilar. The first of these is well 

 exemplified by the solen genus ; in that 

 of the Linnaean tellinx we find examples 

 both of the equivalve shells, and those 

 with the valves slightly different : of the 

 last mentioned kinds we have many ; as 

 the ostrea,spondylus, and anomia. Bivalve 

 shells are often much compressed, some 

 are gibbous, and when viewed at the side, 

 or facing the ligament, have a cordated 

 appearance, as in the venus, and the Lin- 

 naean chama cor. Shells having both valves 

 alike, as before observed, are called equi- 

 valve. Equilateral valves imply those 

 which have both sides of the same valve 

 alike ; as for instance, when a longitudinal 

 line is drawn from the beak to the oppo- 

 site margin, the space on each side of the 

 line is distinguished by the appellation of 

 the right and left side ; and when the 

 form of both those spaces correspond, the 

 shell is equilateral, as in the scallops (os- 

 trea Linn. :) the inequilateral valves are 

 the reverse of this, a line drawn as above 

 described from the beak to the opposite 

 margin, presenting two sides of a very 

 different shape, as we see in most of the 

 mactra, the donax, and tellina genera, 

 and in the mya truncata especially. Sub- 

 equilateral shells, or those having the 

 valves nearly equal at both sides, are suf- 

 ficiently elucidated by shells of the cardi- 

 um, or cockle genus, which are strictly 

 " bivalvis subxquilatera." 



All bivalve shells do not completely 

 close their shells, though most of those 

 before mentioned do so, such as the scal- 

 lop, the donax, tellina, and cardium : in 

 several other tribes of bivalves, when the 

 shells are shut as closely as their form will 

 allow, they still exhibit a kind of hiatus 

 or gaping, either at the anterior or poste- 

 rior end, or at both ; and in some, when 

 the valves are shut, both the anterior and 

 posterior parts are closed, but an opening 



