CONCHIFERA. 431 
terior are called ‘‘ horse-heads” by the Portland quarry-men ;* 
they spoil the stone. Silicified casts have been found at Tisbury, 
_ in which the animal itself, withits gills, was preserved.; 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 very 
uncertain, though they abound in some parts of Sydney Harbour. 
Distribution, 3 species (or varieties ?), Australia. 
Fossil, 100 species. Trias — Chalk (not known in Ter- 
tiaries). Hurope, United States, Chili, Algeria, Cape, South 
India. 
MyornHoriA, Bronn, 1830. 
Type, M. vulgaris, Schl. 
Synonym, Cryptina (Kefersteini), Boue. 
Shell trigonal, umbones turned 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, 16 species. Trias: Germany, Tyrol. 
AXINUS, Sowerby, 1821. 
Type, A. obscurus, Sowerby. 
Synonym, 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 species. Upper Silurian — Muschelkalk. United 
States, Europe. Mactra trigonia, Goldf. Isocardia axiniformis. 
Ph. Anatina attenuata and Dolabra securiformis, M‘Coy, pro- 
bably belong to this genus. Dolabra equilateralis, Amphidesma 
subtruncatum, with many others from the Paleeozoic rocks, may 
constitute a distinct genus, but their generic character has yet 
to be discovered. 
CuRTONOTUS. Salter. 
Thickened hinge-plate, with a single strong triangular central 
* See Plott’s Oxfordshire, T. vii. Fig. 1. 
t In the collection of the late Miss Benett of Warminster, now ir Philadelphia, 
