66 CLASS I. TR0P10P0DA. ORDER I. BIMUSCULOSA. 



UNGULINA, Daudin. 



Testa suborbicularis, rotundata, asquivalvis, subrequilateralis, clausa. 

 Cardo dente in altera valva brevi, subdiviso ; dentibus in altera 

 duobus, quorum unus minutissimus ; cum fovea adjecta, oblonga, 

 marginali, medio angustato-divisa, ligamentum semi-internum reci- 

 piente. Dentes laterales nulli. Impressio muscularis pallii integra, 

 distincte imbuta. 



The genus Ungulina was instituted by Daudin for the reception of a 

 remarkable variety of Mactracea, differing in a peculiar degree from any 

 of the family, particularly in the position of the ligament. 



Its shell is nearly orbicular, rounded, equivalve, subequilateral, and 

 closed at the sides. There is a short, bifid, cardinal tooth in each valve, 

 with the addition of a very small one in one valve only, but no lateral 

 teeth ; and just within the anterior margin, there is an oblong, narrowly 

 divided pit or cavity containing the ligament, which is also divided, and 

 so near the margin as to be partially visible externally. The muscular 

 impression of the mantle is entire and distinctly marked. 



Example. 



PI. XLVI. Fig. 1 to 3. 



Ungulina transversa, Lamarck, Anim. sans vert., new edit., vol. vi. 



p. 122. 

 Ungulina oblonga, var., Deshayes. 



AMPHIDESMA, Lamarck. 



Testa subinsequilateralis, transversa, subovalis vel rotundata, lateribus in- 

 terdum subhians. Cardo aut dente unico, aut dentibus duobus in 



