5IAZATLAN BIVALVES . 13 



Rah. — Panama, in limestone rocks at low water. Cuming. — 

 Mazatlan ; not uncommon ; Upool Col. 



Tablet 38 contains 3 normal specimens. — 39, 3 do. distorted. — 

 40, 2 do. shewing inside, and plates. 



Genus MARTESIA, Leach. 



Differing from Pholadidea in tlie large development of the 

 accessorj^ plates ; from Parapholas in the cups not being per- 

 sistent. 



19. Maktesia intercalata, n. s. 

 M. t. parvd, suhglobos a, in duas partes divisd ; parte 

 aiiteriore tenuissime concentrice striata, radiis longitudinalihiis 

 suhohsoleti^ vlx undatd ; in junior e maxime hi ante, niargine 

 ■folido ; in adultd secretione semi-corned clausd : parte posticd 

 sublcevi, epidermide_ copiose indutd, margine rotundato ; am- 

 honihus testa r^exd adhrerente coelatis ; clypeo pi/riforme, parvo, 

 solido, inter secretiones v alv ar u m inte r c a I a n te ; 

 partihus ventrali et dorsali laminis semi-corneis, hrevihus, a 

 calyce repulsis ; calyce piano, hrevi, simplici, extanie. 



The species is named from the remarkable way in which the 

 nmbonal shield pushes itself in anteriorly between the project- 

 ing portions of the closed valves ; and in wliich the cup, which 

 pouts out from the otherwise rounded extremity, pushes itself 

 in between the antei'ior and posterior plates, cleaving them and 

 thinisting them back. Mr. Hauley was fortimate enough to 

 find two specimens burrowing in Spondylus, of which the larger 

 has not yet closed the anterior portion, but the smaller is adult. 

 The shield does not fold round the dorsal plate. The ^^ hole 

 dorsal and posterior part has a thickened raised margisi, pro- 

 bably epidermal. The largest (adolescent) specimen measures 

 long. '34, lat. '35, alt. '33. The cup in the smaller specimen 

 measures '06. 



Hfd). — Mazatlan ; in Spondylus Lamarckii, extremely rare ; 

 Havre Col. 



Tablet 41 contains various magnified sketches of the two 

 specimens in Mr. Hanley's coUectiou. 



20. . 



A curious little fragment, imfortnnately too imperfect for 

 identification, which may belong to a Pholad, but is certainly 

 distinct from any of the foregoing. Shell with a strong, irre- 

 Aug. 1855. c 



