228 IOWA GEOLOGICAL SURVEY 



plates with convex, upper dental plates with concave or flattened 

 functional surface. As shown by the slope and contour of their 

 inner margin, the two lower dental plates (which correspond 

 to the fused lateral elements in Synthetodus) were not in con- 

 tact with each other along the median line, the resemblance being 

 with Dipterus in this respect. 



The type species of this genus, C. ostraeformis M'Coy, is 

 founded upon a single upper dental plate from the Old Ked 

 Sandstone conglomerate of Scat Craig, Elgin, described by 

 M'Coy in 1848, and stated by him to "resemble the inside of a 

 plicated oyster."* In the same year the genus "Chirodus" 

 was also established by M'Coy upon the evidence of a mandi- 

 bular dental plate with rudimentary plications of the oral sur- 

 face, from the Lower Carboniferous limestone of Derbyshire. 

 There is no valid reason for supposing with Pander that these 

 two forms of Ctenodipterine dentition, separated as they are by 

 a considerable geological interval, are generically identical. 

 The relations, therefore, of the smooth dental plates described 

 by Panderf under the name of Cheirodus jerofejeivi, from the 

 Devonian of Northwestern Eussia, are best expressed by trans- 

 ferring this species to Conchodus, as was first proposed by 

 Smith Woodward. M 'Coy's term is accordingly to be under- 

 stood as a convenient provisional designation for Dipterus-like 

 dental plates in which the usual radiating plications have become 

 atrophied or greatly reduced. 



Now it is interesting to note that the Russian species described 

 by Pander in his monograph on Ctenodipterines, and further 

 illustrated by Eichwald, in his Lethcea Rossica, offers sufficient 

 points of resemblance with the smooth dental plates shown in 

 Plate VIII of the present volume as to suggest the propriety of 

 including them a]l in the same genus, for which it is proper to 

 retain M 'Coy's provisional designation of Conchodus. This 

 procedure is accordingly adopted, and the new specific title of C. 

 variabilis is proposed for the originals of Plate VIII, with the 

 exception of those represented in Figures 16, 20, 29 and 34, 

 which are probably worn examples of Synthetodus. Other exam- 



*Ann. Mag. Nat. Hist. 1848, ser. 2, 2. p. 311. 



t Ctenodipterinen des devonischen Systems. St. Petersb. 1858, p. 33, pi. 6, figs. 

 15-22. 



