124 Dr. L. Hussakof- — U. Devonian Arthrodira from Ohio. 



quite impossible to dispel this doubt since the material necessary to 

 a comparative study of the forms in question is to be found only in 

 America. Last year I enjoyed the privilege of going over tbis 

 material in the British Museum, thanks to the courtesies extended 

 to me bj T Dr. Arthur Smith Woodward, P.R.S., tbe Keeper of Geology, 

 and as I had just come from a study of tbe American forms I was in 

 a position to resolve these doubtful cases. Tbe results of this study 

 are presented in the following pages. 



1. The so-called genus Broxtichthys. 



The name Brontichthys was proposed by Claypole in 1894 l for 

 a large imperfectly preserved mandible having considerable resem- 

 blance to that of TitanicMhys. The type species he named Brontichthys 

 Clarki, in honour of the collector, Dr. William Clark. Clark himself 

 subsequently applied the name Brontichthys, in manuscript, to a pair 

 of small mandibles which he had found associated with some other 

 remains in a small concretion. Plaster casts of this specimen have 

 found their way into most of the larger museums, and the original 

 itself is in the British Museum (P. 9332). 



Pig. 1. Mandible of TitanicMhys sp. Type of Brontichthys Clarki, Claypole. 

 x A. Brit. Mus., P. 9298. Cleveland Shale (Upper Devonian) : Ohio. 



It is obvious that the validity of the genus Brontichthys depends 

 upon the type which was described by Claypole, and only secondarily 

 upon the specimen named by Clark. The former, as stated above, is 

 a large, imperfect mandible (British Museum, P. 9298). A figure ot 

 it is given above (Pig. 1). On careful examination it is seen to be 

 a mandible of TitanicMhys, rather crushed and defective in the anterior 

 extremity — which fact misled Claypole, who regarded it as representing 

 a new genus. It also lacks the posterior extremity. 



As to Clark's specimen (PL VIII, Pig. 2) : this consists of a small 

 block of shale containing a pair of small mandibles, two crushing 

 plates apparently of the upper jaw and not well preserved, and one 

 or two fragments. On close examination the mandibles are seen to 

 pertain to a young individual of Mylostoma variahile, Newberry. 

 At the time Clark applied the name Brontichthys to them, only 

 a few mandibles of Mylostoma were known, namely, those figured 

 by Newberry in his Paleozoic Fishes of North America ; and these 

 were all larger and heavier than Clark's specimen, so that the 

 hitter might then have been looked upon as of a different genus. 

 At the present day, however, several small mandibles of Mylostoma 

 are available with which Clark's specimen may be compared. 



1 " On a new Placoderm, Brontichthys Clarki, from the Cleveland Shale " : 

 Amcr. Geologist, vol. xiv, pp. 379-80, pi. xii. " On a new gigantic Placoderm 

 from Ohio" : Third Ann. Rep. Ohio State Acad. Sci., 1895, pp. 8-9. 



