156 MAZATLAN BIVALVES 
to separate it from the aberrant species from Ld. Hood’s Island. 
The Mazatlan specimens are however distinct both in colour 
and habit of growth. A very young valve ‘15 across is not 
plicated: a larger flat specimen on Crepidula is ribbed, but 
scarcely plaited at the margin: a still larger one is but very 
indistinctly ribbed. A swollen, short specimen, grown on a 
spine of Murex nigritus, is rather strongly plicated ; while the 
largest, in Mr. Darbishire’s collection, grown between two 
folds of Chama, scarcely displays crenations, except near the 
hinge. The finest grown specimen displays the following 
characters: margin scarcely plicate, internally finely crenated 
on each side of the hinge: a deep hollow in each valve running 
up inside the umbos: central teeth (on the attached valve) 
joined together for more than half their height, holding the lig- 
ament, which is extremely small, tubular, only exposed at the 
extremities, and running up to the umbos, though not exposed 
(or scarcely covered) as in Spondylus, but nearer the interior 
of the shell. In the free valve, the ligamental tube rises up, 
separating the pits of the inner teeth. In this specimen the 
muscular scar is almost round; in another, rather oval. The 
valves are held together by the interlocking of the lar ge rugose 
teeth. No other species is described from the West onencan 
eoast. Mr. Darbishire’s specimen measures, long. 1°2, lat. ‘6, 
alt. °38. 
Hab.—Bay of Fonseca, Cuming.—Mazatlan ; extremely rare, 
on shells ; L’pool Col. 
Tablet 701 contains the pair off Murex nigritus; the young 
flat valve; and the finest specimen, off Crepidula aculeata, 
(white var.) 
Famity OSTREID ZA. 
Genus OSTREA, Linn. 
The usual discriminating marks between species are of little 
value in this genus. Neither the shape, amount of adherence, 
sculpture, character of the hinge, colour, direction of the 
umbos, denticulation or plication of the margin, nor even the 
shape of the muscular impression, afford unvarying characters. 
Geographical distribution also is not of much help, the same 
forms appearing in widely distant seas. The study of the 
young shells does not, as itis wont, bring fresh light; very 
widely separated forms being scarcely distinguishable in early 
