Structure and Classification of the Asterolepidx'. 27 



I have seen no trace in Pterichthys of the narrow plate 

 which Pander figured in Asterolejjis (7, tab. vi., fig. 1, No. 2) 

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

 margin of the cranial shiekl in front of the premedian and 

 rii^ht and left laterals. This bone Pander admitted he had 

 never found perfect in the Old Eed Sandstone of Livonia, 

 but thought that he found it i7i situ, in Scottish examples of 

 Fterichthys {ih., tab. vi., fig. 5). Nevertheless, on comparing 

 the figure here quoted with numerous examples of Pter- 

 ichthys, 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. iv., fig. 1, No. 1) I know 

 nothing ; but the oblong plates which he named " Unter- 

 kinnlade " or '' maxilla inferior," are preserved in situ in 

 numerous examples of the Scottish Pterichthys. This sup- 

 posed lower jaw {inn., PL I., Fig. 2) consists of two some- 

 what 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. /.) 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 Asterolepidse, as well as in the Coccosteida?, 

 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 bones, 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 (PL I., Pigs. 1 and 2, represented 

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



