Dr. L. Husmlwf- — U. Devonian Arthrodira from- Ohio. 125 



They agree "well, for instance, with the pair of mandihles figured 

 by Dean l in the specimen showing the associated head and body 

 plates (American Museum, No. 7526) ; they are somewhat smaller 

 than the latter. They may also be compared with a juvenile 

 Mylostoma mandible in the American Museum (No. 3588). Indeed, 

 there can be no doubt that the specimen which Clark referred to 

 Claypole's genus Brontichthys belongs to Mylostoma. 



Prom all this it follows that Brontichthys is not a valid genus but 

 merely a synonym of Titanichthys. 



2. Titanichthys rectus, Claypole* 

 1893. Titanichthys rectus, Claypole, Geol. Surv. Ohio, vol. vii, p. 609, 

 pi. xlii, fig. 5. 



This species was figured and named, but not sufficiently described. 

 It is, however, a good species. The type is a mandible, 23 inches 

 long (Fig. 3), in the collection of the British Museum (P. 9328). It 

 may be briefly described as follows : Mandible as large as that of 

 Titanichthys C'larki, but almost straight fore and aft. Anterior 

 functional portion relatively smaller than the corresponding region in 

 other species of the genus, and not drawn out into an elongate 

 spatulate process, but gradually decreasing in depth to the extremity. 



Fig. 3. Titanichthys rectus, Claypole. Mandible in inner aspect, x J. Type. 

 Brit. Mus., P. 9328. Cleveland Shale (Upper Devonian) : Ohio. 



The species of Titanichthys at present recognizable are four in 

 number — 



T. Agassizi, Newberry, Trans. N.Y. Acad. Sci., vol. v, p. 27, 1885. 



T. Clarki, Newberry, ibid., vol. vi, p. 164, 1887. 



T. attcnuatus, Claypole, Hep. Geol. Surv. Ohio, Tjpl. vii, p. 612, pi. xlii, 



figs. 1, 2, 1893. 

 T. rectus, Claypole, ibid., p. 609, pi. xlii, fig. 5, 1893. 



3. Selenosteus brevis (Claypole). (PI. VIII, Pig. 4.) 



1896. Titanichthys brevis, Claypole, Arner. Geologist, vol. xvii, p. 166, pi. x. 

 1910. Selenosteus Kepleri, Dean, Mem. N.Y. Acad. Sci., vol. ii, p. 94, pis. 



One of the two cotypes of T. brevis, Claypole, has been identified 

 by the writer in the British Museum Collection (P. 9330). It consists 

 of a small slab (PI. VIII, Fig. 4), 23 by 25 cm., Avith its counterpart 

 and several fragments which apparently formed part of the original 

 concretion. It exhibits both mandibles (Mnd.), the median occipital 

 (MO.), and a fragment from the lateral margin of the head (Gran.), 

 the clavicular (CI.), and two or three other plates. The mandibles 

 represent a small species, smaller than any Titanichthys known. 

 It was this circumstance that led Claypole to name it T. Irevis. 



On a careful study of the mandibles it is seen that they do not 

 belong to the genus Titanichthys, but to Selenosteus. They are similar 

 1 Mem. N.Y. Acad. Sci., vol. ii, p. 101, 19^. ^ 



