273 MANUAL OF THE MOLLUSC A. 



Animal with a long and pointed v \i ^ 



foot, bent sharply, heel prominent, sole v ^' ^^^^^^^ 

 bordered by two crenulated ridges; ''' ^^..eTi^^'y^^^^^f^^s. 

 palpi small and pointed; gills ample, ^ d^ l(^/ ^ C^^b^ 



the outer smallest, united behind the ^ Mf^^ " 



body to each other and to the mantle.^ 0' -^ ^^^ m" - i, 



The shell of Trigonia is almost en- S" ^ ^n 



tirely nacreous, and usually wanting or "^^//^ / j^ 



metamorphic in limestone strata; casts ^^^TXr- ^^^ ^^~~ ^ 



of the interior are called "horse-heads'' ^/ '^ "^ '^ .vy^-fv^..-'-- "^T^ 



by the Portland quarry-men;* they unClXCZ^^^'^^ 



spoil the stone. Silicified casts have Fig- 183. Trigonia j>ecUnata.\ 



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. 

 Stutchbiiry on the gunwale of his boat ]eai)t overboard, clearing a ledge of four 

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

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



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



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

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



Myophorta, Bronn, 1830. 



Type, M. vulgaris, Schl. Sijn. Cryptiua (Kefersteinii) Bone. 



Shell trigonal, umbones tm-ned forwards ; obliquely keeled ; smooth or 

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

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

 PL 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. 



Tyj)e, A. obscurus. Shy. Syn. Schizodus, King (not Waterhouse). 



SJiell 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. Silm-ian — 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 PhDadelphia. 



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

 they should terminate near the margin, hetween the arrows which indicata the 

 inhalent and exhalent currents : a, o ', adductors ; Ai, ligament; i. T, dental sockets; 

 0, mouth; U^ labial tentacles or palpi; p, pallial line; m, margin; /, foct; v, cloaca. 



