70 OLD RED SANDSTONE FISHES. 



lateral cranial plate, and arriving in front of the median opening it bends inwards 

 to join the opposite groove on the pre-median plate ; a slightly different arrange- 

 ment, however, is seen in Bothriolepis, as we shall see when that genus comes to be 

 specially considered. 



Classification op the Asterolepidj;. 



The family of Asterolepidae may be briefly defined as follows : 



Head and body covered with osseous plates which are externally sculptured 

 and ganoid, those of the body firmly sutured into a hollow carapace. Eyes 

 approximate, contained in a transverse median orbit, and separated by a loose 

 interocular median plate. Position of the nasal organs not definitely known. One 

 pair of limbs, pectoral, oar- or sword-like, each consisting of many osseous plates 

 enclosing a central tubular hollow, and articulated to the side of the body 

 immediately behind the head. A well-developed lateral sense-canal system. Tail, 

 where known, covered with bony scales, and provided with one dorsal and a 

 completely heterocercal caudal fin. 



The relative fineness or coarseness of the external sculpture is not constant in 

 plates belonging to the same species, for as a rule the ornament is finer in younger, 

 coarser in older individuals. 1 



Four genera — Asterolepis Eichwald, Pterichthys Agassiz, Bothriolepis Eichwald, 

 and Microbrachius Traquair — may with certainty be reckoned to this family, and 

 all occur in British Devonian (Old Red Sandstone) rocks. They are broadly 

 distinguished from each other by the mode of overlapping of the anterior median 

 dorsal plate and the relative length of the arms ; hence the tabulation adopted by 

 Mr. Smith Woodward in his ' Catalogue,' and which may here be quoted : 



I. Pectoral appendages shorter than the body-armour. 



1. Anterior median dorsal plate overlapping (both) the dorso-laterals . Asterolepis. 



2. Anterior median dorsal plate overlapping the anterior dorso-lateral, 



overlapped by the posterior dorso-lateral . . . Pterichthys. 



3. Anterior median dorsal plate in front overlapping and behind over- 



lapped by the anterior dorso-lateral and the posterior dorso-lateral . Microbrachius. 

 II. Pectoral appendages longer than the body-armour. 



4. Anterior median dorsal plate overlapping the anterior dorso-lateral, 



overlapped by the posterior dorso-lateral . . . Bothriolepis. 



Of these genera Asterolepis and Pterichthys resemble each other very closely ; 

 Bothriolepis is in many points of detail considerably different from either ; while 

 the structure of Microbrachius is unfortunately as yet incompletely known. 



1 K. H. Traquair, ' Proc. Koy. Phys. Soc. Edinb.,' vol. xi, 1892, p. 285. 



