146 Bashford Dean Memorial Volume 



of the teeth of Dinichthys shows an intermediate character between the real bone and real 

 dentineJ 



However, the most interesting observation in the study of Arthrodiran teeth was 

 made by Stensio. In 1925 he described a microsection of an Arthrodiran jaw from the 

 upper Devonian of Spitsbergen. According to him "All the teeth are ankylosed to the 

 jaw bones with their bases and generally have the central parts occupied by a rather large 

 pulp cavity. . . . Basally the trabecular-dentine gradually turns into the bone of the 

 mixicoronoid (infero'gnathal). Externally, of the dentine there is in the distal parts of the 

 teeth a layer of bright shining enamel." It is very difficult to correlate this statement with 

 all the others. We can only observe that the infero-gnathal- described by Stensio can not 

 be regarded as typical for the Arthrodira. It belongs, perhaps, to one or another highly 

 specializied Devonian family of the Arthrodira, which has a more complicated tooth struc- 

 ture. Further investigation on this form will show if it can be at all assigned to the 

 Arthrodira. 



Thanks to the kindness of Dr. L. Hussakof, I have had the opportunity of studying 

 the perfect microscopical sections of the "teeth" of Dinichthys in the American Museum. 

 Microphotographs of these sections are to be found in Plate V, figures 10 and 12; and Plate 

 VI, figure 14. In Plate VI, figure 16 there is given for comparison a picture of the minute 

 structure of a carapace plate (intero'lateral) of Dinichthys. As clearly seen there is no 

 radical difference between the structure of the "tooth" and a bony plate. We find the 

 same Haversian canals. Haversian lamellae and osteoblasts in both sections. The only 

 difference is that the structure of the "tooth" is more compact, the Haversian canals are 

 narrower, the lamellae more massive and dense. It is unquestionable that the "teeth" 

 of Dinichthys are formed of true bone. 



All three gnathal elements in Dinichthys are very well known and have been described 

 in detail before by many authors. When first found, they were used as types for many 

 species of Dinichthys. They are most completely described by Newberry (1875 and 1889) 

 and Hussakof (1906). Therefore I shall only give a brief description of them here, point- 

 ing out some special characters in their position and in their relation to one another. 



Antero-Supra-Gnathal. — This plate (Text-figures 27 and 29 ASG) forms the 

 big fore "tooth" of Dinichthys. Its outline and the relation between its single parts vary 

 strongly in different species. But it is always easy to recogni2;e the same elements in its 

 structure. On the front and side this tooth is limited by two nearly plane surfaces A and 

 B (Text-figure 27). They meet at an angle of nearly 90" and thus form a sharp and some- 

 what arched edge of the tooth (Text-figure 27 eg). 



The massive sharp "pick" (Text-figure 27 p) of the tooth is smooth and bright, 

 enamel-like on the outside. On the inside, a clear worn area shows the region where this 

 tooth touches the infero-gnathal (Text-figure 29). In the section near the point, the pick 

 of the tooth is nearly quadrangular (Text-figure 27 IV). Higher up, it becomes more flat 



' Eastman in a description of the dentition of Mylostoma (1906.2) mentioned the presence of dentine and enamel in this form. 

 But Stetson's investigation (1930) of Eastman's original material, shows that there is neither dentiiie nor enamel present. 



