GENERAL CONCLUSIONS 



471 



median or accessory placodes relative to the optic pits for the lateral eyes 

 in vertebrates may be readily explained. 



The presence of the two impressions on the deep aspect of the postero- 

 median plate of Bothriolepis described by Patten has recently (1929-1930) 

 been confirmed by Stensio, who gives an illustration (Fig. 321) of the 

 same two pits in Aster olepis, an allied genus. He suggests that the pits 

 are produced by the attachment of one or several of the recti muscles of 



po 



P''^W J><>V P ek - 



v pp 



Fig. 320. — Dorsal Aspect of the Ocular and Olfactory Plates of Bothrio- 

 lepis ENLARGED. (AFTER PATTEN.) 



A part of the olfactory and rostral plates has been removed on the left in order 

 to expose the deeper-lying sclerotic plates. Between the lateral eyes is the quad- 

 rangular parietal plate, nearly perforated by a deep conical pit opening inward 

 and covered externally by a thin, lens-like tubercle, beneath which was the parietal 

 eye. On the deep aspect of the post-orbital (post median) plate are two similar 

 pits, which Patten believed were occupied by a pair of posterior parietal eyes. 



a.s. pi. : anterior sclerotic plate. 

 le. : lateral ethmoid. 

 Is. pi. : lateral sclerotic plate. 

 me. : mesethmoid. 



0. : corneal opening. 



01. : site of primitive olfactory organ. 

 p.e.t. : parietal eye tubercle. 



po. pi. : postorbital plate. 



p.p. : position of paired pits on inner 



aspect of po. pi. 

 p.s. pi. : posterior sclerotic plate. 

 r. : rostrum. 

 rs. : shelf plate on inner surface of 



rostrum. 



the lateral eye on each side ; a supposition which appears much more 

 probable than Patten's hypothesis ; more especially since the investiga- 

 tions of Stensio and others into the general anatomy of these fishes have 

 definitely proved that the cephalic appendages of Bothriolepis, Asterolepis 

 ornata, and allied genera — which were at one time thought to closely 

 resemble the cephalic appendages of the Merostomata, e.g. Eurypterus 

 — are true pectoral fins, consisting of two segments, each of which 

 contains inside the dermal bony exoskeleton, an axial cartilaginous 

 endoskeleton, which in Bothriolepis was provided with a perichondral layer 



