132 CLASS I. TROPIOPODA. ORDER II. UNIMUSCULOSA. 



TRIDACNA, Bruguiere. 



Testa regularis, aequivalvis, ineequilateralis, lunula hians. Cardo denti- 

 bus duobus in utraque valva compressis, inaequalibus, anticis insertis. 

 Impressio musculi adhrerentis bipartita, subexpansa, prope ad mar- 

 ginem ventralem imbuta. 



The genus Tridacna comes with great propriety at the commencement 

 of this order, because of the duplex or bipartite construction of the ad- 

 ductor muscle ; as it thus exhibits an indistinct modification of the two 

 lateral muscles which characterize the former order. We confess that this 

 affinity is rather forcibly stretched ; still, as we have to lay out the Mollusca 

 in systematic and comparative order, it becomes our duty to analyse, as far 

 as possible, the affinities and relations of the several parts of the system. 

 There most assuredly runs throughout the whole range of animated 

 nature an unbroken link of connexion ; it was a favourite maxim with 

 Linnams, " Natura nonfacit saltum ;" and when we arrive at certain gaps 

 in the chain, or, rather, at certain divisions in the general classification, 

 between which it is difficult to establish an affinity, we can but ascribe 

 the deficiency to the absence of existing forms yet remaining to be dis- 

 covered. The naturalist endeavours, by the aid of certain symbols or 

 characters, to reduce the objects of creation to comparative order ; he 

 establishes a series of artificial divisions of different rank, such as classes, 

 orders, families, and so forth, to enable him to point to any given part 

 of the arrangement ; and delights in the discovery of those forms which 

 appear new, and most likely to diminish the gaps that remain open in a 

 system, which he knows to be all-perfect. 



The shell of Tridacna may be described as being regular, equivalve, 

 disposed in ribs diverging in a longitudinal direction from the umbones 

 to the margin ; and the ribs are often covered with broad high-vaulted 

 scales. The lunula situated on the dorsal part of the shell under the 



