MAZATLAN BIVALVES 15 



strise are coarser and more irregular. This (posterior) portion 

 of the shell is covered with very copious epidermis, in wrinkled 

 folds, as in Saxicava rugosa. Ligament long. The young shell 

 has projecting umbos, and often serrated edges to the anterior 

 gape. The largest specimen measures long. '57, lat. 1'17, alt. 57. 

 The smallest valve, long. '02, lat. '05. 

 Hob. In Spondyli, Isle of Perico, Panama, Cuming. Mazat- 



lan ; not uncommon in Spondylus Lamarckii, very rare in 



Chamse, Patella Mexicana, and in Imperator olivaceus and 



unguis ; Upool fy Havre Coll* 



Tablet 43 contains 6 valves very young, and 3 pairs adolescent. 



44, Young specimen, piercing the apex of Imperator unguis. 



45, do. Imp. olivaceus, with Bryozoa, &c. attached. 46. Young 

 specimen in situ in Imp. olivaceus, with another younger, coter- 

 minal. 47. Young specimen in Imp. olivaceus, with tube some- 

 what projecting, which is unusual at Mazatlan. 48. 1. olivaceus 

 broken across, and shewing 5 specimens of G. truncata, and 

 one Lithodomus caudigerus, invading the Imperator's liver 

 chamber. 49. Adolescent specimen in situ in fragment of 

 Spondylus, with the tube laid bare, and the pedal chink and 

 deposit. Also portions of several other tubes of Gastrochsena, 

 Lithodomus and Petricola. 50, 3 adult specimens varying .in 

 outline, of which one displays a pearl formed near the hinge, 

 another a lamina cutting off a large part of the anterior end, as 

 often happens in large aged fossil Saxicavse. 51. Fragment 

 of Spondylus, displaying hole of G. truncata, which, having 

 approached a Lithodomus too closely, has turned round, filling 

 the vacant space with shelly matter. 52. Siphon pipe laid bare, 

 by the decay of the worm-eaten Spondylus around : also bur- 

 rows of Lithodomus plumula and Parapholas calva. 53, Pipes 

 and thick lining of 3 Gastrochsense in Patella Mexicana : the 

 two siphons in one are ail-but separated at the extremity. 

 54, 2 pipe-ends from Spondylus. 55, Fragments of shelly lining. 



22. GASTBOCHJENA OVATA, Sow. 

 Proc. Zool. Soc. 1834, p. 21. 



Burrow not differing perceptibly from that of G. truncata. 

 Shell, when fresh, of a delicate lilac tinge, with sharp elegantly 

 bent concentric ridges following the margin, closer at the an- 



* In the Bristol Institution is a Tablet marked G. cuneiformis, from the West 

 Indies, Mr. Priske, containing 2 specimens ; of which one I was unable to separ- 

 ate from G. ovata, the other (in its burrow) from G. truncata. Mr. Hanley 

 states, from the examination of a collection made by M. Bean in Guadaloupe, 

 and sent to him by M. Petit, that several species of borers are common to the 

 two oceans. 



