10 MAZATLAN BIVALVES 



forated by otlier borers. The deposit is rarely seen in young 

 shells, but is common in adolescent and ? universal in old speci- 

 mens. At the bottom of the burrow is generally seen an irregu- 

 lar mark projecting beyond the smooth surface of the hole, 

 formed by the Pfoot of the animal. Sometimes this is only a 

 "scar, strongly resembling an irregular oval muscular im^jres- 

 sion with an elevated margiaal ridge from which it sinks back 

 to a deep central linear depression, the whole strongly marked 

 with concentric and radiating furrows." (Darhisliire) . More 

 often there is a sort of side chamber, or shoe, irregularly exca- 

 vated in the shelly matrix, and not always in the same relative 

 position, the siu'face of which is warty and very irregularly 

 corrugated. This is generally filled with a black horny sub- 

 stance, giving an animal smell when burnt, but not displaying 

 any silicious particles. In very old specimens this excavation 

 is often enormously developed, occasionally reaching up be- 

 tween the grey deposit and the shelly matrix. Even -in rather 

 young shells this foot-chink is sometimes seen ; and whenever 

 the burrow reaches the inside of the Spondylus, the black 

 substance is always first apparent. ? Does the foot make this 

 lodgment as a fidcrmn, whUe the valves spin round and form 

 the burrow : and after the adult valves can no longer move. 

 Pdoes the foot amuse itself with forming these superfluous 

 excavations. 



Shell, when extremely yoimg. of xjdophagoid shape, -with the 

 ventral portion imdeveloped and the cup-lamina) only existing 

 as slight folds of the epidermis. Soon however two radiating 

 lines become developed, and the ventral part rapidly increases. 

 Throughout the youuger portion of its life there is a very large 

 dorsal as well as ventral gape. The anterior edge is marvelously 

 thickened, ending in a stout knob, quite capable of aiding in bor- 

 ing execution. In the single specimen found in a ti-ansition state, 

 this knob is no longer prominent, and the anterior gape is par- 

 tially filled with shell, not smooth as in the adidt, but gradually 

 carrying off the ribs of the thicker portion. 



Adult shell squarish or rounded anteriorly, short or length- 

 ened, presenting all the intermediate forms between calva and 

 acuminata of Soiv. ; fitting so tightly into its bxuTOw that when 

 dried it is often impossible to remove it without fractiu-e, the 

 uanbonal plate being firmly adherent to the matrix. The epider- 

 mis is thin on the anterior part ; over the ventral part, folded 

 in thick concentric layers, every 4 or 5 of which (varyuig in 

 number) are as it Mere joined together along the posterior line 

 into a sei-ies of tiles ; and thence continued in a series of over- 



