142 



Bashford Dean Memorial Volume 



orbital impression. Stensio correctly let the sensory canal from PrO continue onto "L" 

 and placed the nostril impressions between ""L" and the rostral. 



Jaekel mentioned this plate in many of his papers and figured it in all his reconstruc 

 tions of Arthrodira from Wildungen (1902, 1906.2, 1907, 1925). According to him, the 

 P7s[ has a deep nostril impression or is even perforated by nostril openings. The sensory 

 canal from PrO is continued on it. He called it in 1902 ""Lacrymale'', later 'Tost-nasale." 



The first author to mention this plate in 

 Dinichthys was Claypole. In his paper on 

 Dimchthys in 1892 (p. 206) he described it 

 briefly, and promised to describe it in detail in 

 a later article, but I have not been able to find 

 this description in any of his papers. In all 

 probability it was never done. Here follows 

 his brief description: 



The pre-maxillary [post-nasal], which 

 has been hitherto unknown, is not found in 

 the specimen here described, but its presence 

 has been established from others. . . . For 

 the present it must suffice to say, that it com- 

 pleted the outline of the fore part of the head, 

 enclosing the nasal openings and filling up the 

 recess observable between the nasal and pre- 

 orbital plates. 



The next author was Branson (1908.1, 

 .2, 1911), who described the post'nasal as a 

 fore part of the sub-orbital and was of the 

 opinion (1908.1, p. 364) that: "The notched 

 anterior end of it (SO) has not been figured or 

 described. ... In the interior of the notch 

 the bone is 15 mm. thick and apparently artic- 

 ulated with some other bone. The anterior 

 slimc'canal seems to be continuous with the anterior slime-canal of the top of the head." 



In the collections of the American Museum, P^i is to be found as a badly preserved 

 specimen and as a plaster cast. The cast is of a big SO, probably of Dinichthys terrelli 

 Nwb., with the post-nasal attached to its front margin. Unfortunately no label is affixed 

 to this cast and nobody can remember when or from what particular locality it came to the 

 Museum. This makes it impossible to find the original, which is necessary for a detailed 

 study. In Text-figure 25 is given a reconstruction of this plate based on the facts known 

 from other Arthrodira, the descriptions and drawings of Claypole and Branson, and, 

 chiefly, on the original pieces and the plaster cast in the American Museum. 



The post-nasal (Text'figure 25 and Plate III, figures 3, 4 and 6) is a little, oblong, 

 fout'cornered plate with the hind margin attached to the front margin of the handle of 



Text-figure 25. 



The post-nasal plate of Dinichthys ? terrelli Nwb. : 



A, from outside; B, from inside. 



ns, nasal impression; pro, contact place with PrO; sc, sen- 

 sory canal connecting pre-orbital and sub-post-orbital canals; 

 so, contact place with SO. 



