THE 



NATURALIST [Vol. XLIII 



postero-ventromedian. In the present specimen we can 

 not prove that some overlap did not occur, for the point 

 of junction is much eroded. We can, however, say that 

 no such overlap is indicated and that the two plates 

 appear to be separated by a thin curved suture which 

 is convex anteriorly and concave posteriorly. 



When the postero-ventrolaterals are compared with 

 those of D. neicberryi ( ?) the general outlines and pro- 

 portions exhibit great similarity, but this is not the case 

 with the antero-ventrolaterals. In these plates not only 

 is there an apparent difference in the posterior over- 

 lapping margins, but the anterior lateral projection is 

 turned posteriorly and not anteriorly as in D. newbemji 

 (?). This gives the entire front margin of the ventral 

 shield an evenly convex outline quite different from the 

 bow-shaped line shown by D. neicberryi ( ?) and in Hussa- 

 kof 's 6 restoration of D. curtus. As a whole the ventral 

 shield exhibits general relations of length to breadth not 

 unlike that of D. neicberryi (?) but is proportionately 

 much broader and stouter than the restored plastron of 

 D. curtus to which reference has been made. 



Comparison with Coccosteus 

 All authorities on the Arthrodira have united in as- 

 signing to Dinichthys halmodeus a primitive position 

 among American Dinichthyids and the species is re- 

 garded as having diverged only slightly from the an- 

 cestral genus Coccosteus. These relationships have been 

 made out by studies on the cranium, the infero-gnathals 

 and on the dorsal body plates. 



If then the specimen here considered is indeed specif- 

 ically identical with D. halmodeus we have further con- 

 firmation of the practically intermediate position which 

 the species holds between Coccosteus on the one hand 

 and the highly specialized species of Dinichthys on the 

 other. 



6 L. Hussakof. On the Structure of Two Imperfectly Known Dinich- 

 thyids. Bull. Am. Mug. Nat. Hist., Vol. XXI, Art. XXV, p. 412. 



