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. 
Famity GASTROCHZANIDZ. 
Genus GASTROCH ZENA, Spengler. 
21. GASTROCHENA TRUNCATA, Sow. 
Proc. Zool. Soc. 1834, p. 21. 
Animal excavating a smooth oval chamber at the end ofa 
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 & Hanl. 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°5in, exclusive of the cell. 
