CONCHIFERA. 



711 



prominent, they are said to be cordiform. 

 When they are inclined towards one another, 

 so that their summits approximate, they are 

 said to be opposed. The hooks in no case in- 

 cline to the posterior side; but occasionally 

 they disappear almost entirely, and, as in the 

 Solens, exhibit no kind of prominence. In 

 other instances again they project a great way, 

 and form the most prominent part of the shell 

 (Mytilus, Pinna), in which case the hooks are 

 said to be terminal. 



2. The belli/ of the shell comprises the 

 greatest part of the exterior surface. It is 

 bounded at the base of the hook, as also by 

 the lunule and the corslet. It is more or less 

 rounded or flattened according to the general 

 form of the shell, and we shall speak of the dif- 

 ferent points worthy of consideration con- 

 nected with it when we come to define the 

 various particulars of the external surface con- 

 sidered in a general manner. 



Fig.36Q A. 



Cf: 



a, superior edge ; b, nncus ; c, Innula ; e, anterior 

 edge ; /, inferior edge ; g, posterior edge. 



JYg.368 B. 



a, anterior edge ; b, inferior edge ; c, posterior 

 edge ; d, edges of the shield ; /, ligament ; 

 f to h, nympha ; h, other extremity of the liga- 

 ment ; i, point of the uncus or hook ; / to n, 

 lunula; I, anterior cardinal tooth; j, median 

 cardinal tooth ; g, posterior cardinal tooth j m, 

 anterior lateral tooth ; o, anterior muscular im- 

 pression ; p, posterior muscular impression ; r, 

 palleal impression ; s, sinuosity of the palleal 

 impression. 



3. The edges. -- These are indicated, pre- 

 serving the shell in the position which we 

 have mentioned; they are anterior, posterior 

 inferior, and superior. The extent of these 

 edges is very various, and depends entirely 

 on the form of the shell and the position of its 

 hooks. Upon this particular the simple in- 

 spection of a collection of shells will give 

 much more information than we can hope to 

 do by the most laboured description ; so that 

 we shall only say that the anterior edge cor- 

 responds to the head of the animal, the pos- 

 terior to its posterior extremity, the inferior to 

 its ventral aspect, and the superior to its back. 

 The edges in themselves, however, present a 

 few particulars which it were well to mention. 

 Sometimes they are thin and cutting ; very com- 

 monly too they are thick and continue simple. 

 In those species especially whose shells are 

 marked externally with longitudinal stria, they 

 are notched and toothed alternately, the pro- 

 jections of the one valve in almost all instances 

 being received into the cavities of the other. 

 When these projections and notches are very 

 fine, the shell is said to be crenate; if larger 

 toothed; when very large and few in number 

 with their summits blunt, again, the edge is 

 undulating as in the Tridacna; on the con- 

 trary it is angular when the prominences con- 

 tinue sharp as they do in certain of the Ostreae; 

 in the latter case the edge is also said to be 

 widely or deeply dentated. 



4. The lunula does not occur in every 

 genus of bivalve shell. It is met with, how- 

 ever, among the greater number of the Mono- 

 myaria ; it is also met with among many Dy- 

 myaria, and is particularly conspicuous in the 

 Venus. It is a space comprised on the ante- 

 rior surface immediately under the hooks, and 

 is generally circumscribed by a particular line 

 or depression. The lunula presents certain pe- 

 culiarities which it is often of consequence to 

 attend to, in order to distinguish certain spe- 

 cies otherwise apt to be confounded with one 

 another. Its form is various, sometimes cordi- 

 form, a shape which almost peculiarly belongs 

 to inflated and subglobular species; sometimes 

 lanceolate, sometimes very narrow, especially 

 in species whose shells are flattened. The 

 lunula is very rarely prominent, unless it be 

 towards the centre. Sometimes it is super- 

 ficial, pretty frequently depressed, and there 

 are a few genera or species in which it is deeply 

 hollowed. 



5. The corslet occupies a part of the su- 

 perior and posterior edge of the shell. It is 

 only met with in the Dimyaria ; it is not so 

 accurately bounded as the lunula ; it is also 

 wanting in a great number of genera, in which 

 its presumed position is arbitrarily determined. 

 It is towards its upper part that the nymphae 

 are observed in those species whose ligament 

 is external. 



In a very considerable number of Mono- 

 myarians the lunula and corslet are replaced 

 by certain projecting parts to which the name 

 of auricula or auricles has been given. These 

 occur more especially in the Pecten family 

 the Pectinites of Lamarck ; they are distin- 



