622 



GEOLOGY OF OHIO. 



under side of the body. As they are apparently assignable to no other 

 place in the armor of Dinichthys I provisionally locate them here." (P. 

 138). 



Ventral Armor of Coccosteus decipiens, Ag. Pander's restoration. 



But while the vacancy in the plastron in not positively filled, another 

 series of four bones is described which is believed to have defended the 

 under side of the head. These are a pair of "jugulars" fitting into the arched 

 space between the mandibles, and a pair of "post jugulars" or "hyoids" 

 which overlapped the jugulars at their tips. The description is as fol- 

 lows: "They [the jugulars] are each semi-elliptical in outline, sixteen 

 inches in length by seven and a half wide ; the outer margin is sym- 

 metrically arched ; the inner margin nearly straight ; the posterior ends are 

 obliquely truncated and overlapped by the anterior extremity of a second 

 pair of plates. * * * The posterior pair of jugulars — or as they should 

 perhaps be called hyoid plates — are long- triangular in outline, smaller 

 than the anterior pair, but much thicker. Their anterior angles overlap 

 and are sunk into the obliquely truncated ends of the jugulars. The out- 

 side and posterior ends of the hyoid plates are irregular and thin and 

 show that they were overlapped by other plates." [Monograph p. 137-8.] 



The valuable material in t::y hands now enables me to show, as I 

 think beyond a doubt — 



First, that the "post jugular" or "hyoid" bones refered to above are 

 in reality the anterior ventrals of the plastron, but with their apices 

 turned in the opposite direction. 



