488 Dr. E. H. Traq-uair on the 



under the name of " os terminale," as forming the anterior 

 margin of the cranial shield in front of the premedian and 

 right and left laterals. This bone Pander admitted he had 

 never found perfect in the Old Bed Sandstone of Livonia, 

 but thought that he found it in situ in Scottish examples of 

 I'terichthys {ib. tab. vi. fig. 5) . Nevertheless on comparing the 

 figure here quoted with numerous examples of Pterichthys I 

 am satisfied that the suture there indicated as marking off the 

 "os terminale" is only the transverse groove, belonging to the 

 lateral line system, which crosses the front part of the head. 



Of the small, narrow, doubly curved ossicle designated by 

 Pander " Oberkiefer " (7, tab. vi. fig. 1, no. 1) I know 

 nothing ; but the oblong plates which he named '^ Unter- 

 kinnlade " or " maxilla interior " are preserved in situ in 

 numerous examples of the Scottish Pterichthys. This sup- 

 posed lower jaw [mn. PI. XVII. fig. 2) consists of two 

 somewhat narrow oblong plates, meeting each other in the 

 middle line and placed transversely on the under surface of 

 the head right in front of the semilunar plates [s. I.) of the 

 ventral body-carapace. Each is narrower at the outer than at 

 the inner end and somewhat concave above ; frequently they 

 occur displaced forwards, even to a position altogether in 

 front of the head. These plates may indeed have formed the 

 inferior (or posterior) boundary of the mouth ; but it is clear 

 that their mode of working must have been rather different 

 from that of the mandible of ordinary Vertebrata. 



Before leaving the head of Pterichthys it may be well to 

 point out the distribution of the lateral line system on this 

 part. In all the Asterolepidaj, as well as in the CoccosteidEe, 

 this system consists of grooves, which are apt to be, and have 

 often been, mistaken for sutures, actual or obsolete; but they 

 do not occur on the inferior surfaces of the boneSj and their 

 connexion with a similar groove running along each side of 

 the body amply demonstrates their true nature. In Pterich- 

 thys each cephalic groove (PI. XVII. figs. 1 and 2, repre- 

 sented by the double dotted lines) passes from the dorso- 

 lateral plate of the body on to the external occipital, where 

 it at once bifurcates, a transverse branch passing across over 

 the posterior part of the median occipital to join its fellow of 

 the opposite side. The main groove then runs forward on the 

 lateral cranial plate, and, arriving in front of the median 

 opening, it bends inwards to join the opposite groove on the 

 premedian plate, on which it forms a small backward flexure. 

 This course is altogether similar to that in Asterolepis ; but, 

 as we shall see, it is in some particulars ditferent from the 

 arrangement seen in Bothriolejjis. 



