256 



SHELLS AND SHELL-FISH. PART I. 



the beautiful analogy which is found to exist in the 

 two typical tribes of the Dithyra and the Gasteropoda, 

 and is not borne out by the details of their analysis. 



(240.) The Macrotrachia and the Atrachia com- 

 prise by far the greater number, and the most perfect, 

 of all the bivalves : and that they each form a circular 

 group, will be apparent from the following consider- 

 ations. The Macrotrachia may be said to have their 

 first developement in the family oi PhoUdcp, — several 

 of which, as Teredo, Clavagella, &c,, assume the form 

 of the Tahulihranchia. By means of Pholas and its 

 sub-genera, there is a direct passage to the Myad^ 

 through Solen : these semitubular shells are succeeded 

 by Mya and Mactra, where all appearance of the 

 gaping peculiarity of the Myadce ceases. Vi,^e enter 

 the Tellinidce by means of the river genera allied to 

 Cyclas; and thus, through Venus, reach the TeJMncp, 

 where the two siphons of the animal are of enormous 

 length. Cardissa Sw. and Pleurorynchus of Phillips 

 open the passage to the Chamid^, where we see the 

 siphons much reduced, and almost obsolete in Hippopus 

 and Tridacna. The aberrant genera of Cardita and 

 Chama unite with Cypricardia, and lead to the perfo- 

 rating family of the Saxicavid^, into which also 

 enter Petricola, Saxicava, Coralophaga, and Thracia. 

 Finally, all these are as evidently connected to Gale- 

 omma, as Galeomma is to Gastrochina. Thus we 

 again arrive among the Pholadcp, and the circle of 

 the Macrotrachia is closed. 



(241.) The course of the Atrachia is precisely 

 similar. Cuvier has already pointed out the connection 

 of the ChamidcE to the Unionidcp; while the genus 

 Iridina in the latter, and Nucula in the Arcadje, show 

 that the two families follow each other. The passage 

 from these to the AvicuLin^ is still more perfect,upon 

 looking to the genera Byssourca and Mod'wla: then 

 follow Crenatula, Malleus, Avicula, and Pinna, which, 

 uniting to Modlolii, completes this group. Avicula 

 opens the passag? to the Ostreid.*;, which are entered 



