96 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



discussion a survival of the primitive conditions observed in Mesacan- 

 ttius, with the exception that the free spines between the paired fins 

 have become lost. Whereas in most Acanthodians the dorsal exceeds 

 the anal more or less in size, in this case it is the anal that is slightly 

 the larger. 



The squamation is very fine indeed, and on this account the individual 

 granules can scarcely be recognized. The lateral line, however, is very 

 distinctly shown in both examples. In the larger specimen the calcified 

 meckelian cartilage is preserved on one side, and is substantially of the 

 same form as shown by Jaekel ^ and E.eis ^ in A. bronni. The smaller 

 specimen exhibits a displaced " extramandibular spine " with long fine 

 rays, such as occurs in A. bronni and A. {Traquairia) pygyncea. Teeth 

 are absent. By an odd chance a small Pleuracanthus tooth has hap- 

 pened to lodge directly over the posterior part of the cranium in the 

 larger specimen, thus increasing the obscurity of this region. The com- 

 pact structure of the fin-spines and mandibular calcifications leaves no 

 room for doubt that these are adult individuals, notwithstanding their 

 small size. 



The species is named in honor of Professor Charles E. Beecher as a 

 slight tribute of personal esteem, and in grateful appreciation of his 

 many friendly courtesies. 



Formation and Locality. Coal Measures ; Mazon Creek, Illinois. 



1 Jaekel, 0., Ueber die primare Zusammensetzung des Kieferbogens und Schul- 

 tergiirtels. Verhandl. deutsch. zool. Ges. (1899), p. 252, text-fig. 1. — Zeitschr. 

 deutsch. geol. Ges., Verhandl., Vol. LI., 1899, p. 56, text-fig. 1. 



2 Reis, 0.. M., loc. cit., Plate VI., Figs. 1, 3, 4. 



