1910.] NATURAL SCIENCES OF PHILADELPHIA. 659 



Dinichthys Eastman, 5 Dean 6 and Yon Koenen 7 have described more 

 or less complete specimens of the ventral covering with its elements 

 in nearly the positions occupied during the life of the animal. In 

 these cases the several species of Dinichthys, to which the ventral 

 shields have been referred, are typical examples of the genus. The 

 ventral armor of the primitive species usually included in Dinichthys 

 (such as D. halmodeus) has until recently remained unknown. 



In the summer of 1908 the writer was so fortunate as to obtain a 

 specimen of the ventral armor of a small arthrodire from the Marcellus 

 shale (Middle Devonian) of central New York. Although no definite 

 specific determination has been attempted, it is highly probable that 

 the specimen in question represents the plastron of Dinichthys hal- 

 modeus (Clarke). Though originally described as Coccosteus, 8 most 

 recent authors have included this species in Dinichthys, all agreeing, 

 however, that it holds a position only slightly removed from the 

 former ancestral genus. The chief reasons for assigning this armor 

 to D. halmodeus are (1) similarity of superficial tuberculation, (2) 

 geographical and geological position, and (3) the fact that this ventral 

 shield possesses just such primitive characters as would naturally 

 correspond with those exhibited by the dorsal plates and head shields 

 of the type material of the species. 



A detailed description of this ventral shield has been given else- 

 where, 9 and for our present purposes it is sufficient to say that the 

 plates are in their original position, the antero-ventromedian and 

 postero-ventromedian, both postero-ventrolaterals, and one antero- 

 ventrolateral are practically entire, while the other antero-ventro- 

 lateral is mostly retained. The outline of this latter plate can, however, 

 be restored from that of its well-preserved fellow on the other side. 

 In addition to these ventral plates the specimen shows both the 

 antero-dorsolaterals and the right postero-dorsolateral. Two other 

 bones associated with those already named were originally regarded 

 as a probable suborbital and as a possible antero-superognathal. A 



5 C. R. Eastman, "On the Relation of Certain Plates in the Dinichthyids," 

 Bull. Mus. Comp. Zool., Harvard, vol. XXXI, pp. 26, 27, pi. I, fig. 2, and pi. IV. 



6 Bashford Dean, "On the Vertebral Column, Fins, and Ventral Armoring 

 of Dinichthys," Trans. N. Y. Acad. Sci., vol. XV, pp. 157-163, pis. VII and VIII ; 

 "Note on the Ventral Armoring of Dinichthys," Trayis. N. Y. Acad. Sci., vol. 

 XVI, pp. 57-60, pi. III. 



7 A. von Koenen, "Ueber einige Fischreste des norddeutschen und bomischen 

 Devons," Abhandl. k. Gesell. Wiss., Gottingen, vol. XL. 



8 John M. Clarke, "New and Rare Species of Fossils from the Horizons of the 

 Livonia Salt Shaft," Rep. State Geologist, New York. 1893, p. 161. 



9 Burnett Smith, "On Some Dinichthyid Armor Plates from the Marcellus 

 Shale." Am, Nat., vol. XLIII, Oct., 1000.' 



