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 elUptica, pi. Ixxii, figs. 23-33 ; pi. xcv, fig. 18. 

 Paracyclas lirata, pi. Ixxii, 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 Russia 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. Right 

 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 



