130 CLASS I. TROPIOPODA. ORDER II. UNIMUSCULOSA. 



Acuities that ever oppose our ingenuity to reduce the affinities of nature 

 to an arbitrary division. The first two families of this order, the Tri- 

 dacnacea and the Mytilacea, are said to be provided with a second muscle, 

 distinctly figured both by Poli and Quoy, though very small and closely 

 connected with the other. Deshayes, indeed, goes so far as to assert, that 

 Lamarck might as well have included them with the Bimusculosa, and 

 that the Tridacnacea are intimately allied to the Chamacea ; we cannot, 

 however, reconcile ourselves to this arrangement, because the animal of 

 the former is somewhat opposed to that of the latter, being entirely 

 reversed in its shell, with the foot passing out through the lunular open- 

 ing under the umbones. We propose to consider this small additional 

 muscle merely as an accessory cartilage, destined to strengthen the 

 central muscle. The Tridacnacea are animals requiring great muscular 

 power, and the Mytilacea may require an accessory ligament to coun- 

 teract the powerful action of the hinge ligament, with which their shell 

 is provided in the absence of teeth. The Aviculacea, too, appear in some 

 instances to possess several of these small accessory cartilages : in the 

 shell of the Avicula margaritifera (vide PI. CX.) a series of impressions of 

 attachment may be distinctly traced running from the seat of the central 

 muscle towards the hinge ; and as in these several instances the animal 

 is furnished with a beard or byssus, for the purpose of attaching itself to 

 different marine bodies, may it not be also inferred, that these accessory 

 cartilages are destined to assist the foot in fixing or displacing it '? But a 

 modification of this character may be traced in many of the Bimusculosa ; 

 in the family of the Naiades, for instance, the muscular impression is said 

 to be compound, when the shell exhibits any of these small accessory 

 marks of attachment. 



Lamarck includes the Brachiopodous Mollusca in this order ; but sub- 

 sequent discoveries have confirmed the anticipations of Cuvier, that they 

 have an organization essentially different. The elaborate investigation 

 of these animals by Professor Owen has decided their claim to a separate 

 and particular class. 



The Unimuscular Tropiopoda, which are far less numerous than those 

 of the former order, are divided into five families, as follows : 



