18 



MAZATLAN BIVALVES 



nately copied tliis description into Lis Cat. Ten. B. M., instead 

 of the original one. The young is generally (so Phil.) "sub- 

 globosa, alba," and occasionally shews the radiating ribs "pos- 

 tice distantibus" ; the bifidity of the anterior tooth in the left 

 valve is rarelj^ 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, Hlac 

 and chocolate, developing at last into a lustrous brown, especi- 

 ally at the muscular impressions. MeanwhUe the characteristic 

 concave decussating strise 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 iip into knobs. 

 The anterior ventral part is generally much swoUen, often with 

 a concave 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 

 Unguliaa) is turned up spirally round the umbos, as inChamfe, &c. 



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, ii*regular, and always exposes 

 to view the extremities of both valves. Mr. Darbishire has 

 one foiu'th of a Spoudylus valve, with, no fewer than eleven 

 Petricolse projecting their extremities. The interior of the 

 burrow is irregular and rough. One of the Chamse, here- 

 inafter enumerated, displays a Petricola, which, having bored 

 through the Chama and come to the rock, prefei-red flat- 

 tening its own valve to undertaking the unexpected labour. 

 Tliis creature, havkig no power to move roimd 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 hoi'ny substance as in Parapholas. 



The largest specimen measures long. '87, lat. 1'22, alt. '73, 

 The most oval „ " -63, „ '78, „ -52, 



The smallest „ „ -06, „ '08, „ -04. 



Hah. — In rocks, 6 — 11 fm., Panama and Isle Muerte, Cuming. — 

 Jun. in Avicula margaritifera, Panama, E. B. Philippi. — 

 Mazatlan : in Spondylus Lamarkii, not uuconmion : very 

 rare in Chama. Veiy young shells are rarely met with ; 



