MAZATLAN BIVALVES 13 
Hiab.—Panama, in limestone rocks at low water, Cuming.— 
Mazatlan ; not uncommon ; L’pool 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 the large development of the 
accessory plates; from Parapholas in the cups not being per- 
sistent. 
19. MaRTEsiaA INTERCALATA, 7. S. 
M. t. parvé, subglobosd, in duas partes divisd; parte 
anteriore tenuissime concentrice striata, radiis longitudinalibus 
subobsoletis vie undatéi; in juntore maxime hiante, margine 
solido ; um adultd secretione semi-corned clausd : parte posticd 
sublevi, epiderinide copiose indutd, margine rotundato; um- 
bonibus testd reflexa adherente celatis ; clypeo pyriforme, parvo, 
solido, inter secretiones valvarum intercalante; 
partibus ventrali et dorsali laminis semi-corneis, brevibus, a 
calyce repulsis ; calyce plano, brevi, simplici, extante. 
The species is named from the remarkable way in which the 
umbonal shield pushes itself in anteriorly between the project- 
ing portions of the closed valves ; and in which the cup, which 
pouts out from the otherwise rounded extremity, pushes itself 
in between the anterior and posterior plates, cleaving them and 
thrusting them back. Mr. Hanley was fortunate 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 whole 
dorsal and posterior part has a thickened raised margin, pro- 
bably epidermal. The largest (adolescent) specimen measures 
long. 34, lat. °35, alt. °33. The cup in the smaller specimen 
measures ‘06. 
Hab—Mazatlan ; in Spondylus Lamarckii, extremely rare ; 
Havre Col. 
Tablet 41 contains various magnified sketches of the two 
specimens in Mr. Hanley’s collection. 
IpitEs 
A curious little fragment, unfortunately 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. ¢ 
