MAZATLAN BIVALVES 



is strong, curling round the umbos, like Isocardia, and becom- 

 ing semi-internal beyond the hinge teeth. All the specimens 

 are dextral. The laminse are generally finely striated outside. 

 These shells, like those of Spondylus, offered a safe retreat 

 for boring and nestling bivalves and numerous minute Gas- 

 teropods. The smallest valve measures '02 across. That with 

 the largest animal, (shell somewhat thin,) long. 4*5, lat. 4*25. 

 alt. 3 '4. The heaviest shell measures externally (though part 

 has been removed) long. 5*7, lat. 5', alt. 5' ; internally (from 

 ligament to margin) long. 2*6, lat. 2 '8, alt. 2' 2. This, with 

 another smaller specimen attached, weighs 51b. ; and displays 

 a very large and persistent spiral ligamental groove. 

 Hal). Gulf of Tehuantepec, Mexico ; dredged up from sandy 



mud attached to Aviculse, 10 fm. ; Mus. Cuming. Mazatlan ; 



not common ; L'pool fy Havre Coll* 



Tablet 425 contains 3 valves, extremely young, exhibiting 

 the shagreened surface at the umbos. 426, 2 do. umbos finely 

 striated. 427, 1 do. umbo spiral, as in young Calyptrseadse. 

 428, 1 do. concentric foliations developed. 429, 8 valves, a 

 stage older, spiny processes developed, valves inside variously 

 coloured, and surface rugose. 430, 1 pair and 3 valves, a stage 

 older, colour highly developed. 



Tablet 431 contains 1 pair young. 432, a specimen with the 

 spiral umbos so enormously developed, as to approach the fos- 

 sil form Diceras. The umbo of the attached valve makes two 

 loosely spiral whirls, the ligament running up the suture ; in 

 length it nearly equals the breadth of the shell, but being 

 filled up with solid matter, would not display itself in the 

 internal cast. 



Tablet 433 contains a pair, young, purple and orange, (closely 

 resembling the C. echinata, "very old," figured in Beechey's 

 Voyage.) Margin very finely, and teeth strongly crenate. 

 Outside, with Vermetida3, Lithophagi, &c. 434, 2 valves at- 

 tached to each other, pierced by Lithophagi, &c^ The outer 

 layer has become eroded, except where the Lithophagi have 

 bored. 



Tablet 435 contains a diseased specimen, purple, attacked by 

 Lithophagi, even to the very centre of the teeth. 



Tablet 436 contains a finely grown adult specimen, attached 

 to Pinna, with Vermetidse &c. 437, the sp. with the largest 

 animal. 438, the group with the largest shells. 



* A specimen received from Dr. Gould of this species (without reference) is 

 probably the "C. Pacifica" of his list, "Guaymas, Lieut. Green." It may 

 however be the C. Buddiana, C. JB. Ad. : v. infra. 



