CHAP. II. GREAT CIRCLE OF THE TESTACEA. 47 



family as much diversified as any of the naked Testacea, 

 but which^ at present^ remains almost as a genus. In 

 it are found representations of nearly all the onisciform 

 Mollusca, as Sigaretus, Chelisoma, Chelinotus, Chiton, 

 and numerous others : the interesting genus Tristoma 

 of Cuvier, contains the first rudiments of branchia^ — for 

 such do we consider that " ramified circular vessel in 

 the parenchyma of the body/' the nature of which^ as 

 that learned anatomist conceives, "^ it is difficult to 

 determine."* This, together with the many beautiful 

 and interesting forms discovered and figured by Riip- 

 pell, establishes the union of the PlanaridtB and the 

 Dor idee — in other words, the two orders of Parenchy- 

 mata and Nudibranchia — in the most perfect manner. 

 We pass, then, from these latter, to the Branchiopoda, the 

 first tribe in the order of bivalves. From this point our 

 course is plain : Anemia connects them with the Ostraa 

 and other Dithyra without siphons ; while these latter 

 are connected to the more typical bivalves, where these 

 organs are fully developed, by means of Chama and 

 Hippopus. We quit the perfect bivalves for such as 

 are tubular, and hence almost univalve shells, through 

 the Myadce and SolenidcE, and thus reach the Teredincp, 

 By this latter family, the path is smoothed to the 

 terminal series of the Dithyra, — namely, the Tuhuli- 

 hranchia of Cuvier, — where we have the singular union 

 of a gastropod mollusk inhabiting a tubular shell. 

 The two typical orders being thus united, we enter, at 

 once, among the spiral or ies,td.Qeou^ Gasteropoda, — that 

 group, in short, which stands at the head of the entire 

 class. Beginning with the Scutihranchia or Patellides, 

 as the least organised of these univalve animals, we see a 

 gradual developementof the spire take place in the Halio- 

 tida', Trochido', and Helicidce ; until, in the Turbidce, 

 we have a union of the phytophagous and the zoophagous 

 gastropods. The old genera of Melania in one, and 

 Cerithium in the other, effects this union, and conducts 

 us at once to the Strombidce, MuricidcB, and, finally, to 



* Griff. Cuv. xii. 473. 



