1g MAZATLAN BIVALVES 
nately copied this description into his Cat. Ven. B. M., mstead 
of the original one. The young is generally (so Phil.) “sub- 
elobosa, alba,” and occasionally shews the radiating ribs “pos- 
tice distantibus’”’; the bifidity of the anterior tooth in the left 
valve is rarely seen in the adult. When extremely young, the 
shell is irregularly rugose, but not shagreened : the apex is then 
moderately prominent: the rugosities soon fall into regular: 
ribs dividing between the posterior and ventral parts; the 
interspaces are finely rugose. Very soon the apex disappears, 
and.a large space round becomes eroded. The shell which has 
previously been white, assumes various shades of orange, lilac 
and chocolate, developing at last into a lustrous brown, especi- 
ally at the muscular impressions. Meanwhile the characteristic 
concave decussating strie appear between the ribs; and on the 
posterior ribs (which are generally closer than the ventral but 
not so close as the anterior ones) are gathered up into knobs. 
The anterior ventral part is generally much swollen, often with 
a coneave margin between that and the posterior end, which is 
not unfrequently much produced, flattened, and even bent up- 
wards. The short, semi-internal ligament (somewhat resembling 
Ungulina) is turned up spirally round the umbos, as inChame, &e. 
The burrow is shaped as the shell, with a rising corresponding 
with its posterior ventral depression, and without any siphon 
gallery. The opening is oval, irregular, and always exposes 
to view the extremities of both valves. Mr. Darbishire has 
one fourth of a Spondylus valve, with no fewer than eleven 
Petricole projecting their extremities. The interior of the 
burrow is irregular and rough. One of the Chama, here- 
inafter enumerated, displays a Petricola, which, having bored 
through the Chama and come to the rock, preferred flat- 
tening its own valve to undertaking the unexpected labour. 
This creature, having no power to move round in its hole, is 
frequently bored into by its neighbours. A pedal scar is occa- 
sionally seen towards the anterior end, sometimes deep, with 
dried horny substance as in Parapholas. 
The largest’ specimen measures long. 87, lat. 1°22, alt. °73, 
The most oval he Fla 63, hee AO oe en Oe, 
The smallest " ee OOF ane: goa. ee 
Hab.—tIn rocks, 6—11 fm., Panama and Isle Muerte, Cuming.— 
Jun. in Avicula margaritifera, Panama, #. B. Philippi — 
Mazatlan: in Spondylus Lamarkii, not uncommon: very 
yare in Chama. Very young shells are rarely met with: 
