DEVONIAN FISHES OF IOWA 87 



marked that the appearance of these plates does not even re- 

 motely suggest comparison with the dentition of Arthrodires, 

 and that structures corresponding to Dipnoan vomerine teeth 

 do not occur in any known Asterolepid. 



The pattern of the cephalic sensory canal system is consider- 

 ably different from that in Asterolepis and Pterichthys. No 

 transverse commissure unites the lateral canal of each side across 

 the occipital plates; but in front, just at its incurved flexure 

 on the lateral plate, a conspicuous branch is given off, which 

 runs forwards and outwards to the margin of the shield, being 

 very likely continuous with that on the maxillary plates on the 

 lower side of the headshield. On the median occipital two 

 fainter canals are observable, forming an angle with each other 

 but not always uniting behind, and extending outwards and 

 forwards over the lateral plates until they become confluent, or 

 almost confluent, with the lateral canal near its central point of 

 flexure. Near the same point, on the inner side of the main 

 groove, a small ear-shaped mark is often, but not always to 

 be seen. The main groove is continued nearly parallel with the 

 periphery until it reaches the center of the premedian plate, 

 where it is sometimes interrupted for a short distance. 



The articulations of the body plates are as in Pterichthys, the 

 anterior median dorsal (a. m. d.) overlapping the antero-dor- 

 solateral {a. d. I.) of either side, but being itself overlapped by 

 the postero-dorsolaterals (p. d. I.). On the under surface (text- 

 fig. 13) the place of the two semilunars of Pterichthys is filled, 

 according to Professor Patten, by at least three pieces, the 

 posterior margins of which are assumed by this author to have 

 been ' ' freely movable in a dorso-ventral direction, like an oper- 

 culum."* The course of the lateral sensory canals is exactly 

 the same as in the body plates of Pterichthys, but in addition two 

 shallow linear grooves diverge at a slight angle from about the 

 center of the antero-dorsomedian plate, and extend across the 

 postero-dorsolaterals. 



The pectoral appendages are longer than the dorsal aspect of 

 the body armor, and even pass beyond the termination of the 

 ventral surface. The proximal portion is also longer than the 



* Patten, W., Structure of the Ostracoderms. Science, n. s., 1903, 17, p. 489. 



