14 MAZATLAN BIVALVES 



o-ularly curved tooth, deeply channeled on the convex side, 

 appressed to the iimbo and projecting considerahly above the 

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

 and apparently mth groove for external ligament. Outside 

 oval, smooth near imibo, afterwards ^\ath slight concentric 

 rounded ribs and very slight radiating furrows. 

 Tablet 42 contains the fragment, and a magnified sketch. 



Family GASTEOCH^NIDJE. 

 Genus GASTROCH.^NA, Spengler. 



21. Gasteochjsna tetjncata, Sow. 



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



Animal excavating a smooth oval chamber at the end of a 

 flattened narrow gaUery, occasionaUy five times as long as 

 the chamber itself:* The gaUery is straight, variously bent, 

 or even suddenly recurved at a considerable angle, according 

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

 evidently different from the shelly matrix, but separable 

 fi-om it The interior of the siphon gaUery is more or less 

 rugose, and towards the extremity is nearly bipartite 1 he 

 nresence 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 ascertamed. When tlie> 

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

 approaching stranger even at a distance, they fiU their cbscarded 

 burrow witl 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 m ajiy 

 ^azatlan spechnen. A ? foot mark is often observed opposite 

 tlantSio'ga^e, but never large as.in Parapholas : occa- 

 sional hi the Adolescent sheU, a chmk is seen in a downward 

 direction, fiUed with the black horny substance 



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

 form var£ even m the young shell and often, both in outlme 

 and markings, approaches ovata. J^may be known a^^^^^^^^^^ 

 however by the great peculiarity of the l^iige; m Mhich the 

 '• sma 1 spathulat^e lamina" which Forbes ^' Ilanl ^^ave record- 

 ed in the r generic character, though scarcely observable m G^ 

 mo oliiia, assumes dimensions which ahnost entitle it and 

 Zuar species to subgeneric rank. Umbos very near anterior 

 g^cTai indistinct diagonal Imc beyond which the concentric 

 • Mr. DarblsWre found one ^yllich measured 4- 6 in. exclusive of the ceU. 



