MA2ATLAN BIVALVES 7 



CLASS PALLIOBRANCHIATA, Blainv. 



Brachiopoda, Cuv. Brachiopoda and Rudista (pars) Lam. 

 Palliobranchiata, Blainv. 



FAMILY DISCINTD^S. 

 GENUS DISCINA, Lam. 

 Discina, Lam. 

 Orbicula, Sow. not Cuv. or Lam. 



-$* 14. DISCINA CUMIN an, Brod. 



Orbicula Cumingii, Brod. in Proc. Zool. Soc. 1833, p. 125. 

 Brod. in Trans. Zool. Soc. i. 143, pi. 23. f. I Mull. Syn. Test. 

 Viv. p. 175. Lam. An. sans Vert. Desk. ed. tert. iii. pt. i. p. 

 118. no. 3. Sow. Thes. Conch, p. 366. no. 4. pi. 73. f. 6 

 D'Orb. Voy. Amer. Merid. Moll. p. 677. B. M. Cat. of 

 D'Orb. Shells, no. 786, p. 89. 



Discina Cumingii, Davidsons Class, of Brack, in Brit. Fos. 

 Brack, vol. i. p. 129. 



Tliis shell is the Pacific analogue of D. striata, which pro- 

 bably belongs to the Atlantic. Some specimens of each species 

 are hard to distinguish from the other. D. Cumingii varies 

 greatly in form, according to the place of its attachment, and is 

 often extremely thin, and scarcely shelly. This is always the 

 case with the lower valve, which has its disk of attachment 

 subtsrmirial, covered with a thin skin except at a very small 

 chink. The upper valve is generally smooth near the apex, 

 which is occasionally subcentral, afterwards closely set with 

 radiating striae, more or less developed, and generally (not 

 always) decussated by slightly raised lines of growth. Some- 

 times nearly the whole of the upper valve is smooth and thin, 

 as in Isevis. The lower valve generally displays only the con- 

 centric lines of growth, but is occasionally decussated near the 

 edge. It is sometimes so convex as to allow scarcely any space 

 for the animal. Muscular impressions not so strongly marked 

 as in D. striata. The young shells are often fringed round the 

 edge, (with the remains of the cilia, Brod.) Colour, brownish 

 yellow. The largest valve measures, ?o^. '4tf,alt. 'OS in. 

 Hob. Payta (Peru), St. Elena, Panama, attached to lower sides 

 of stones in sandy mud at low water 6 fm. : Cuming. Payta, 

 Ecuador, St. Elena, D' Orbigny Seas of Chili [?] 'and Peru, 

 Deshayes. Panama, common, under stones at low water, 

 C. B. Adams. Mazatlan ; on Ostrea iridescens and Virginica, 

 Chama% Pinnse, Spondylus Lamarckii, Strombus galea, in um- 

 bilical chink of Yitularia salebrosa ; rare, L'pool Sf Havre Coll. 



