492 Edward Phelps Allis jr., 



the Placodermi (Zittel), however, so-called lateral sensory grooves are 

 frequently desci'ibed. These grooves are found on the outer surface 

 not only of the bones that form the cranial shield of the head, but 

 also on that of the bones that form the body carapace. They are 

 said to often be very conspicuous, and they must be relatively deep 

 and narrow, for those on the head have often been mistaken for 

 suturai lines separating certain of the bones of the cranial buckler. 

 The grooves, in this, resemble the canals of the chondrostean rather than 

 those of the holostean ganoids, as they do also in that the anastomoses 

 of the different grooves always take place near the centres of the 

 cranial bones, and not between them. The grooves might however 

 have lodged unenclosed, and not enclosed sensoiy lines, the sensory 

 grooves of these fishes then resembling the slight groove that under- 

 lies the middle head line of pit organs of Esox, and the sensory 

 system representing the same, apparently primitive, condition that is 

 found in the deep-sea fishes Lophius and Chaunax. And the assumption 

 that the pit lines, as well as the canal lines, of the holostean ganoids, 

 are represented in the grooves of the Placoderms, permits a compari- 

 son of the lines in the two groups of fishes. 



In Coccosteus [7?], Dinichthys [54, pi. 52] and Titanichthys [54, 

 p. 117] three sensory grooves ladiate, on each side, from the central ; 

 part of the top of the head, exactly as the three dorsal head lines of 

 pit organs radiate from the same region of the head of Amia. The 

 arrangement in Dinichthys and Titanichthys is so strikingly similar to , 

 that in Amia that it would seem as if the grooves in the ones must 

 be the homologues of the pit lines in the other. The point from which 

 the grooves radiate in these fossil fishes should then be occupied by a 

 parietal bone, as it is in Amia. Assuming this to be so, the frontal \ 

 bone of Newberry's \p4, p. 52] descriptions of Dinichthys would become || 

 a parietal, and his parietal would become the mesial one of two extra- 

 scapular bones found on each side of the head. On this mesial extra- 

 scapular bone, a groove, which must be the supratemporal groove, 

 begins, and runnin ; laterally and backward joins the main canal on 

 the so-called epiotic, which latter bone is thus a lateral extrascapular. 

 Precisely the same arrangement is presented by both Titanichthys and 



