10 MAZATLAN BIVALVES 



forated by other borers. The deposit is rarefy seen in young 

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

 mens. At the bottom of the burro ; v 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 impres- 

 sion with an elevated marginal ridge from which it sinks back 

 to a deep central linear depression, the whole strongly marked 

 with concentric and radiating furrows." (Darbiskire) . 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 surface 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. P Does the foot make this 

 lodgment as a fulcrum, while 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 young, of xylophagoid shape, with the 

 ventral portion undeveloped and the cup-laminse only existing 

 as slight folds of the epidermis. Soon however two radiating 

 lines become developed, and the ventral part rapidly increases. 

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

 dorsal as well as ventral gapa 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 transition state, 

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

 tially filled with shell, not smooth as in the adult, 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 Sow. ; fitting so tightly into its burrow that when 

 dried it is often impossible to remove it without fracture, the 

 umbonal 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 (varying in 

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

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



