MAZATLAN BIVALVES | 
CLASS PALLIOBRANCHIATA, Blamv. 
Brachiopoda, Cwv. Brachiopoda and Rudista (pars) Lam. 
Palliobranchiata, Blainv. 
Famity DISCINID/. 
Genus DISCINA, Lam. 
Discina, Lam. 
Orbicula, Sow. not Cuv. or Lam. 
14. Disctina Cuminett, Brod. 
Orbicula Cumingii, Brod. in Proc. Zool. Soc. 1833, p. 125.— 
Brod. in Trans. Zool. Soc. 1. 143, pl. 23. f. 1— Mill. Syn. Test. 
Viv. p. 175.— Lam. An. sans Vert. Desh. ed. tert. 1. pt. 1. p. 
118. no. 3.—Sow. Thes. Conch. p. 366. no. 4. pl. 73. f. 6.— 
D Orb. Voy. Amer. Merid. Moll. p.677—B. M. Cat. of 
D Orb. Shells, no. 786, p. 89. 
Discina Cumingii, Davidson's Class. of Brach. in Brit. Fos. 
Brach. vol. 1. p. 129. 
This 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 
subterminal, covered with a thin skin except at avery small 
chink. The upper valve is generally smooth near the apex, 
which is occasionally subcentral, afterwards closely set with 
radiating striz, 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 levis. 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, long. °47, alt. *08 in. 
Hab.—Payta (Peru), St. Elena, Panama, attached to lower sides 
of stones in sandy mud at low water—6 fm.: Cuming.—Payta, 
Eeuador, 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, 
Chame, Pinne, Spondylus Lamarckii, Strombus galea, in um- 
bileal chink of Vitularia salebrosa ; rare, L’pool & Havre Coll. 
