OF CONCHOLOGY. 147 



This species would seem, from the figures of Messrs. Davidson 

 and Crosse, to be quite distinct from C. Woodwardiana, yet it 

 is possible that one of them may inadequately represent the 

 shell, and the two prove identical. In this case, Mr. Davidson's 

 name has a few weeks' priority. 



It is singular that the descriptions of these last mentioned 

 species should have been first published, almost at the same 

 moment, by different authors, especially as new species of Mega- 

 thyrn are by no means common. 



? Family THECIDIIDJE. 

 Thecidan, Desh., En. M. iii, 1836. 



Shell (perforate when young ?) attached by the neural valve 

 when adult. 



• Brachia lobed, not spiral, shell articulated by teeth, sockets, 

 and a cardinal process. 



Disk grooved or excavated for the reception of the brachial 

 lobes. Mantle adherent, extending to the margin of the valves ; 

 the brachia folded upon themselves and supported by a compli- 

 cated calcareous network secreted by the mantle. Loop absent, 

 except the crura, which usually are united behind the mouth, 

 forming an arch over the viscera ; but are occasionally blended 

 with the surrounding calcareous matter. 



Soft parts with the mantle united behind. Sexes separated. 

 Female provided with a nidamental sac, in which the embryos 

 are incubated, after passing out of the perivisceral cavity through 

 the oviduct, and becoming attached to two specially modified 

 brachial filaments, the ends oi which are inserted in the marsu- 

 pium. Intestine csecal. Mantle margin without setae. Em- 

 bryos transversely segmented, covered with vibratile cilia, pro- 

 vided with two or four ocular spots on the anterior segment, and 

 strongly resembling the embryonic Pneumodermon. Adults sen- 

 sible to the light. Perivisceral fluid colorless. 



Genus THECIDIUM (Defr.), Sby. 



Thecidea, Defr., 1828, in Fer. Tab. Syst. p. 38. 



Thecideum, Lacaze-Duthiers, Ann. Sci. Nat. Zool., T. xv, 



1861, p. 262. 

 Thecidium, Sby. (emend.), Gen. Shells 1844, xx. 

 Thecidium, Dav., Int. Brit. Brach. p. 77 ; Mem. Lin. Soc. de 



Normandie 1856, p. 145. Suess, Ibid. p. 45. Deslongch., 



Ibid. p. 61, and most modern authors. 



Shell thick, punctate, rounded or oval, more or less regular. 

 Neural valve somewhat produced and perforated by a minute 



