10 MAZATLAN BIVALVES 
forated by other 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 ?foot of the animal. Sometimes this is only a 
“sear, 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.” (Darbishire). 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. ? Does the foot make this 
lodgement as a fulerum, while the valves spin round and form 
the burrow: and after the adult valves can no longer move, 
?does the foot amuse itself with forming these superfluous 
excavations. 
Shell, when extremely young, of xylophagoid shape, with the 
ventral portion undeveloped and the cup-lamine only existing 
as sight 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 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 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- 
