MAZATLAN BIVALVES 141 
This species, belonging to the group of B. barbata, setigera, 
&c., appears peculiar in the structure of the epidermal lamelle, 
in which are imbedded the rather strong hairs that run down 
each of the faint radiating grooves, presenting an appearance 
like a bat’s wing, or the whalebones of an umbrella. At 
regular intervals over the surface is a row of larger hairs; 
those on the posterior part are long and stiff, curling round 
almost into a tube. Only one specimen was found in the 
boxes : a very few others obtained from a shop were probably 
from the same collection.— Long. °76, lat. 1°3, alt.°57. 
Hab.—Mazatlan; extremely rare; L’pool Col. 
Tablet 651 contains the specimen. 
193. Byssoarca 1LLoTa, Sow. 
Proc. Zool. Soc. 1833, p. 18. 
Arca illota, Rve. Conch. Ic. pl. 12, £.78.— Hanl. Rec. Shells, 
pl. 18, f. 41. 
Compare A. Tabogengis, C. B. Ad. Pan. Shells, p. 262, no, 424. 
Shell, as usual, varying in form and markings ; but generally 
with extremely fine sharp radiating ribs, and fainter concentric 
ones decussating on a smooth surface ; the ribs being nodulous 
at the sides, and in the adult on the whole surface, when they 
become much larger. Epidermis as in B. Tabogensis, between 
pilose and lamellar, except on the ligamental area, where it is 
smooth and shining. Shape generally of the B. lactea type, 
but with very close umbos, ineequilateral, and flattened at the 
ventral margin. Sometimes this is considerably incurved. 
Hinge teeth few, rather small, in two unequal, slightly curved 
branches. Ligament solid, adhering only at the posterior part, 
whence it makes a fine curl round the umbos. A few large 
specimens were obtained from shops (probably from this col- 
lection) but only three small ones from the boxes. A rounded 
specimen measures long. °64, lat. °9, alt. °55. 
A produced sp. » weno, Mele.” "20 nS, 
Hab.—Gulf of Nicoya, under stones, Cuming.—Do. Hinds, 
B. M.—Mazatlan ; extremely rare ; L’pool Col. 
Tablet 652 contains a small pair from Mazatlan; and a large 
valve from a shop. 
194, Byssoarca GRapaTa, Brod. § Sow. ($24. 
Arca gradata, Zool Journ. vol. iv. p. 365.—Gray in Zool. 
Beech. Voy p. 152, pl. 43, f.1.—Rve. Conch. Ic. pl. 14, f. 92,— 
