126 MAAZTLAN BIVALVES 



is the pattern constant, though there is a general adherence to 

 a particular plan. It often presents the appearance of grains 

 of detritus cemented together by animal matter, these grains 

 also covering the body of the shell much more coarsely than 

 in L. aristatus. It is thus that the young shells can generally 

 be separated ; the epidermal tubercles being much larger, and 

 the concentric wrinkles stronger. The number of specimens 

 however was not large enough to ascertain these points with 

 precision. The differences in the pattern of incrustation do 

 not accompany those in form. The shape presents the same 

 two extreme varieties, gracilior and tumidior, as inL. aristatus. 

 The parallelism of the margins also is not constant. The bur- 

 rows are in most respects like those of L. aristatus, q. v. They 

 are shaped like the shell, and allow very little space for the 

 opening of the valves. The orifice is almost close to the ex- 

 terior, and is not bilobed. The largest perfect specimen 

 measures long. 1'86, lat. '53, alt. '5. A very tumid sp. in 

 Mr. Darbishire's Col. measures long. 1'9, lat. '9, alt. '6. 

 Sal). Panama, in Spondyli, Cuming. Philippines [?], Han- 

 ley. Mazatlan ; rare in Spondyli, very rare in Chama3 and 

 Patella Mexicana ; IS pool fy Havre Col. 

 Tablet 572 contains 6 pairs young, the smallest "08 in 

 length. 573, 4 sp. adolescent and adult, typical form. 574, a 

 sp. in situ, burrowing in the attached part of the Spondylus 

 calcifer : portions of the rock remain, which appear to have 

 been tfcred by Pholads ; a burrow is seen of P. calva. 575, 

 2 fraglBnts shewing the extremities of the burrows which are 

 more nearly round than in L. aristatus. 576, fragments illus- 

 trating shell structure. 577, do. incrustations. 



Tablet 578 contains 3 sp . different ages, var. gracilior. 

 Tablet 579 contains 1 pair and 2 valves do., var. tumidior. 

 Tablet 580 contains a sp. with the anterior part less swollen, 

 and the lines not parallel. 



+ 



4- 176. LlTHOPHAGUS AETSTATUS, Sol. 



Mytilus aristatus, Dillw. Descr. Cat. 1817, vol. L p. 303, no. 8. 



Solander, ms.Wbod, Ind. Test. pi. 12, f. 8. 

 Lithodomus aristatus, Forbes fy Hani, Br. Moll. vol. ii. p. 212. 

 Enc. Meth.Vers, pi. 221, f. 8, a, b. 

 Le Kopan, Adans. Sen. p. 267, pi. 19, f. 2. 

 Mytilus lithophagus striatus, J. Sow. Linn. Trans. 1804, 



vol. viii. p. 274, pi. 6, f. 2, 3-5. 



