490 Dr. E. H. Traquair on the 



ventro-lateral plate of A. ornatus in tlie British-Museum 

 collection. And as regards Sir Philip's appeal to Pander's 

 figures 5 and 9 on tab. v. of his work, in which the " tho- 

 racic " plates seem to be represented in specimens of Pferich- 

 thys from Lethen, he could not surely have read the author's 

 explanation of these figures, in which it is expressly stated 

 that this appearance is due to fracture ! 



Nevertheless, accepting Pander^'s description of these parts 

 to hold good for Asterolepis and Egerton's for Pterichthys^ 

 Bejrich (10), Lahusen (11), and Zittel (16) have sought 

 herein to find a diagnostic mark between the two genera ; 

 but this idea T cannot corroborate. Careful study of a large 

 scries of Scottish examples of Ptericlithys has convinced me 

 beyond all doubt that Egerton was in error on this point and 

 that his ''thoracic" plates are simply parts of the anterior 

 ventro-laterals, separated not by a sufure, but by an internally 

 projecting ridge, which, in crushed and decorticated speci- 

 mens, gives the false impression of a division. I may add 

 that the species macrocephalus, in connexion with which Sir 

 Philip expressed his o])inion so strongly, is not a Pterichthys^ 

 but a Bothriolepisj and that isolated pi ;tes of the larger species 

 of the same genus demonstrate absolutely the unity of the 

 anterior ventro-lateral and the position upon it of the pectoral 

 articulation. 



The articular fossa on the outer side of the anterior ventro- 

 lateral in Ptericlithys^ with its contained helmet-process 

 grasped by tlie articular plates of the arm, and the foramen 

 for the passage of the vessels and nerves to the same, seems 

 to be conformed exactly as in Aaterohiyis ; and as these parts 

 have been so well described by Pander from Russian speci- 

 mens of the latter genus, it is needless at present to enter 

 into detail resjjccting them. If the Scottish and Russian 

 genera are distinct the diagnosis must be founded on some- 

 thing else than the articulation of the limbs. 



Thirdly, as to the articuhition of the body-plates with each 

 other. Sir Philip Egerton states that "all the plates of the 

 carapace, with the exception of the lozenge-sha])ed plate g 

 (of the under surface), are united by simple sutures; this, on 

 the contrary, is attaciied to its neighbours by broad squamous 

 suturts, the lateral bones overlapping its margins on all sides " 

 (8, p. 806) ; but in the same paper he quotes Hugh Miller 

 to the effect that the two median dorsal plates overlapped some 

 neighbouring ones and were themselves overlaj)ped by others. 

 Now my observations show that all the plates of the carapjice 

 were connected with each other by overla[)ping or squamous 

 sutures, a marginal band along the internal surface of the 



