COJ«CHlFERA. 431 



terior are called " liorse -heads " by the Portland qtlarry-men;* 

 they spoil the stone. Silicified casts have been found at Tiabury, 

 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 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). Europe, United States, Chili, Algeria, Cape, South 

 India. 



MyoPhoeia, Bronn, 1830. 



Type, M. vulgaris, Schl. 



Synonym, Cryptina (Kefersteinii), 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, PI. XYII., Pig. 25, has 

 a lateral tooth at the dorsal angle of the left valve* 



Fossil, 16 species. Trias : Germany, Tyrol. 



Axmus, 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 Dolahra securiformis, M'Coy, pro- 

 bably belong to this genus. Dolahra equilateralis, Amphidesma 

 subtruncatum, with many others from the Palaeozoic rocks, may 

 constitute a distinct genus, but their generic character has yet 

 to be discovered. 



CtJETONOTtrs. Salter. 

 Thickened hinge-plate, with a single strong triangular central 



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



X In the collection of the late Miss Benett of Warminstel', now in PlnladelpMa. 



