MAZATLAN BIVALVES 135 
interstitia haud equantibus; in valvad majore planatis, 
subobsoletis, interstitiis minimis ; ligamento solido, aream rhom- 
boideam implente ; dent. card. in lined curva, parvis ; margine 
valve majoris effosso, valvamn alteram recipiendo ; costibus prop- 
ter tenuitatem intus monstrantibus. 
The above description is written from three beautiful ex- 
amples in the Museum of Hugh Cuming, Esq. fragments 
only of young shells were found in the L’pool collection, which 
however probably belong to the same species. It is known 
from A. incongrua and its congeners by its light structure, 
smooth epidermis, and especially by the ribs which are very 
narrow on one valve with wide interstices, on the other broad 
and flat, scarcely divided. There is a corresponding difference 
in the crenations of the valves. Long. 1°55, lat. 1°73, alt. 1°32. 
Hab.—Mazatlan, Mus. Cuming.—Do ; fragments on Spondylus 
ealeifer ; L’pool Col. 
Tablet 631 contains the fragments. 
184. ARCA TUBERCULOSA, Sow. 
Proc. Zool. Soc. 1833, p. 19.—Miill. Syn. Test. Viv. p. 179.— 
Phil. Abbild. I. pl. 1, f.2.—Rve. Conch. Ic. pl. 3, f. 18.— 
Hanl. Descr. Cat. p. 161, pl. 18, f. 538.—C. B. Ad. Pan. 
Shells, p. 263, no. 425. 
Comp. Area similis, C. B. Ad. Pan. Shells, p. 262, no. 422. 
Shape varying from subquadrate, subequilateral, to very 
transverse, with the posterior part much produced ; sometimes 
flattened, sometimes extremely swollen. Ribs very numerous 
(about 36), close, slightly and irregularly tuberculous, often 
shewing concentrie ridges of growth. Epidermis dark brown, 
coarse, deciduous near the umbos, with short scaly hairs in 
the intercostal spaces, and sometimes finer hairs on the pos- 
terior part. Inside white: teeth numerous, rather broad. 
Ligament coarse, solid, entirely filling up the rather narrow 
area between the beaks. Hinge line sometimes rounded off. 
The largest specimen measures long. 2°78, lat. 3°67, alt. 2°5. 
A squarish PF be ee DF i; Boo, ) a eee 
A swollen 2 i HRN TE EARN i) 3 1s (eee 
Hahb.—Real Llejos, at roots of Mangrove trees, low water, 
Cuming.—Panama, in impalpable mud, under a Mangrove 
thicket, near high water mark, not uncommon, C. B. Adams, 
—Mazatlan ; very common; L’pool & Havre Coil. 
