SYNOPSIS OF GENERA. 
XXXIX 
This fossil possesses many of the external characters of the finely striated 
forms of .modern Lucina; and the distinguishing characters are not strongly 
marked. 
The original specimen on which the genus was founded is somewhat vertically 
compressed; giving it an elliptical form. In well-preserved specimens the length 
is but little greater than the height. The genus is very distinctly limited, and 
usually easy of recognition among the Devonian forms of this class of fossils. 
Examples: Paracyclas elliptica, pi. lxxii, figs. 23-33 ; pi. xcv, fig. 18. 
Paracyclas lirata, pi. lxxii, figs. 2-19 ; pi. xcv, fig. 19. 
Paracyclas tenuis, pi. lxxii, figs. 20-22; pi. xcv, fig. 25. 
Schizodus, King (Annals and Magazine of Natural History, vol. xiv, p. 313. 
1844; Geology of Ilussia and the Ural Mountains, vol. ii, pp. 308-9. 1845 ; 
Monograph of the Permian Fossils of England, pp. 185-8, pi. xv. 1850). 
[Type, Schizodus truncatus, King.] 
“ Diagnosis. Equivalved, inequilateral, the posterior side being the longest. 
Anterior outline rounded; posterior tapering toward the extremity. Eight 
valve with two smooth cardinal teeth ; left valve with three teeth. The teeth 
of the left valve placed in front of those in the right valve. The central tooth 
of the former more or less bifid, according to species, on its free side, and 
embraced by both teeth of the opposite valve. Pallial line entire; smooth or 
ornamented with fine raised lines, running parallel to the margin.” 
This genus was proposed, but not described, by Prof. King in the Annals 
and Magazine of Natural History, as above cited, in 1844, to include cer¬ 
tain forms of the Permian and Carboniferous formations which had before 
been referred to Axinus. The diagnosis given by De Verneuil in Geology of 
Russia, etc., above cited, Mr. King regards as incomplete, and the results of 
his own observations are given above. 
The Devonian species of New York correspond in external characters with 
the typical species of King, and in three of these we have been able to deter¬ 
mine a corresponding hinge structure, and muscular impressions. The external 
