156 MAZATLAN BIVALVES 



to separate it from tlie abcrraut species from Ld. Hood's Island. 

 The Mazatlan specimens are liowever distinct both m 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 hold together by the interlocking of the large rugose 

 teeth. No other species is described from the West American 

 coast. Mr. Darbishii-e's specimen measures, long. 1'2, lat. '6, 

 alt. -38. 



Hah. — Bay of Fonseca, Cumi?i</. — Mazatlan ; extremely rare, 

 on shells ; L^'pool Col. 



Tablet 701 contains the pair off Murex nigritus'; the young 

 flat valve ; and the finest specunen, off Crepidula acideata, 

 (white var.) 



Family OSTEEID^. 



Genus OSTREA, Linn. 



The usual discriminating marks between species are of little 

 value in this gi-nus. Neither the shape, amount of adherence, 

 sculpture, character of the hinge, colour, direction of the 

 umbos, denticidation 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 it is wont, bring fresh light ; very 

 widely separated forms being scarcely distinguishable in early 



