MAZATLAN BIVALVES 7 



CLASS PALLIOBRANCHIATA, Blainv. 



Bracliiopoda, Cuv. Bracliiopoda and Eiidista (pars) Lam. 

 Palliobrancliiata, Blainv. 



Family DISCtNTD.^. 

 Genus DISCINA, Lam. 



Discina, Lam. 



Orbicula, Sow. not Cuv. or Lam. 



14. Discina Cumin gii, Brod. 

 Orbicula Cnmingii, Brod. in Proc. Zool. Soc. 1833, p. 125. — 

 Brod. in Trans. Zool. Soc. i. 143, pi. 23. f. I.— Mull. S//n. Test. 

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

 118. no. d.—Smv. T/ies. Conch, p. 366. no. 4. pi. 73. f. 6.— 

 D'Orb. Vol/. Amer. Merid. Moll. ^.677.— B. M. Cat. of 

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

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

 Brack, vol. i. p. 129. 



This sbell is the Pacific analogue of D. striata, whicLi pro- 

 bably belongs to the Atlantic. Some specimens of each species 

 are liard 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 

 subtermbial, covered M-ith a thin skin except at a verj^ 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 gi-owth. Some- 

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

 as in Itevis. The lower valve generally disi^lays 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 imi^ressions 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. Tlie largest valve measures, Zo«</. '^7, alt. '08 in. 

 Hah. — Pajiia (Peru), St. Elena, Panama, attached to lower sides 

 of stones in sandy mud at low water — G fin. : Cuming. — Payta, 

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

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

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

 Chama-, Pinns, Spondylus Lamarckii, Strombus galea, in um- 

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



