14 MAZATLAN BIVALVES 



gularly curved tooth, deeply channeled on the convex side, 

 appressed to the umbo and projecting considerably above the 

 margin of the valve. No process on the hinge as in Pholas, 

 and apparently with groove for external ligament. Outside 

 oval, smooth near umbo, afterwards with slight concentric 

 rounded ribs and very slight radiating furrows. 

 Tablet 42 contains the fragment, and a magnified sketch. 



FAMILY GASTBOCHJENID^E. 

 GENUS GASTROCHLENA, Spengler. 



*J- 21. GASTEOCH^NA TEUNCATA, Sow. 



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



Animal excavating a smooth oval chamber at the end of a 

 flattened narrow gallery, occasionally five times as long as 

 the chamber itself.* The gallery is straight, variously bent, 

 or even suddenly recurved at a considerable angle, according 

 to circumstances. It is always lined with a shelly coat, not 

 evidently different from the shelly matrix, but separable 

 from it. The interior of the siphon gallery is more or less 

 rugose, and towards the extremity is nearly bipartite. The 

 presence of these creatures may always be detected by the 

 8 -shaped holes they make ; but there is no clue by which the 

 probable place of their abode can be ascertained. When they 

 make a sudden turn, in consequence of some knowledge of an 

 approaching stranger even at a distance, they fill their discarded 

 burrow with shelly lining. The English species has no objec- 

 tion to burrowing through the graves of departed brethren, 

 Carrying its tube across them: this was not noticed in any 

 Mazatlan specimen. A ? foot mark is often observed opposite 

 the anterior gape, but never so large as in Parapholas : occa- 

 sionally in the adolescent shell, a chink is seen in a downward 

 direction, filled with the black horny substance. 



The valves have free room to open widely in the cavity. The 

 form varies, even in the young shell, and often, both in outline 

 and markings, approaches ovata. It may be known at once 

 however by the great peculiarity of the hinge ; in which the 

 "small, spathulate lamina" which Forbes 8f Hani, have record- 

 ed in their generic character, though scarcely observable in G. 

 modiolina, assumes dimensions which almost entitle it and 

 similar species to subgeneric rank. Umbos very near anterior 

 gape : an indistinct diagonal line beyond which the concentric 



* Mr. Darbishire found one which measured 4' 5 in. exclusive of the cell. 



