272 MANUAL OF THE MOLLUSCA. 



Animal with a long and pointed 

 foot, bent sharply, heel prominent, sole 

 bordered by two crenulated ridges; 

 palpi small and pointed ; gills ample, 

 the outer smallest, united behind the 

 body to each other and to the mantle.^ 



The shell of Trigonia is almost en- 

 tirely nacreous, and usually wanting or 

 metamorphic in limestone strata; casts 

 of the interior are called "horse-heads" 

 by the Portland quarry-men;* they 

 spoil the stone. SHicified casts have Fig- 183. Trigonia pectinata.% 

 been found at Tisbury, in which the animal itself, with its gills, was preserved. f 

 The species with the posterior angle of the shell elongated, have a siphonal 

 ridge inside. The epidermal layer of the recent shell consists of nucleated 

 cells, forming a beautiful microscopic object. A Trigonia placed by Mr. S. 

 Stutchbury on the gunwale of his boat leapt overboard, clearing a ledge of four 

 inches ; they are supposed to be migratory, as dredging for them is ver 

 uncertain, though they abound in some parts of Sydney Harbour. 



Distr. 3 sp. (or varieties ?) Australia. 



Fossil, 100 sp. Trias Chalk; (not known in Tertiaries). Europe 

 U. S. Chile, Algeria, Cape, S. India. 



MTOPHORTA, Bronn, 1830. 



Type, M. vulgaris, Schl. Syn. Cryptina (Kefersteinii) Boue. 



Shell trigonal, umbones turned forwards ; obliquely keeled ; smooth o 

 sculptured ; teeth 2.3, striated obscurely , centre tooth of left valve simple 

 anterior of right valve prominent; mould like Trigonia. M. decussata 

 PI. XVII. fig. 25, has a lateral tooth at the dorsal angle of the left valve. 



Fossil, 13 sp, Trias : Germany, Tyrol. 



i AXINUS, Sowerby, 1821. 



Type, A. obscurus, Sby. Syn. Schizodus, King (not Waterhouse). 



Shell trigonal, rounded in front, attenuated behind ; rather thin, smooth 

 with an obscure oblique ridge ; ligament external ; hinge-teeth 2.3, smooth 

 rather small ; anterior adductor slightly impressed, removed from the hinge 

 with a pedal scar close to it ; pallial line simple. 



Fossil, 20 sp. U. Silurian Muschelkalk. U. States, Europe. Mactra tri 



* See Plott's Oxfordshire, T. vii. fig. 1. 



t In the collection of the late Miss Benett of Warminster, now in Philadelphia. 



J Fig. 183. From a specimen in alcohol ; the gills slightly curled and contracted 

 they should terminate near the margin, between the arrows winch indicate th 

 inhalentand exhalent currents : a,a ', adductors ; hi, ligament; 1. t', dental sockets 

 o, mouth; It, labial tentacles or palpi; p, pallial line; m, margin; /, foot; v, cloaca. 



