36 PISCES. 



The cranial roof is usually covered with small dermal plates, 

 while the orbit is surrounded by a ring of four circumorbitals. 

 There are no remains of a gill-cover, which must thus have 

 been only membranous if present ; but each gill-arch bears on 

 its hinder or convex border a close series of lanceolate dermal 

 appendages, which may be either gill-rakers or the supports of 

 frills like those of the modern Chlamydoselache. Each of the 

 fins, except the caudal, is provided with an anterior spine, 

 which resembles that met with in the dorsal fins of many well- 

 known Selachians, and is to be similarly regarded as an 

 enormous dermal ray. A paired series of similar spines is also 

 present in some of the earliest genera between the pectoral and 

 pelvic fins, and it has been suggested that these are remnants 

 of the once-continuous lateral fin-folds (fig. 28). Some hard 



FIG. 28. 



Climatius scutiger ; outline of fish, with spines shaded, slightly enlarged. 

 L. Old Red Sandstone ; Forfarshire. a, anal fin-spine ; d 1 , d 2 , dorsal fin- 

 spines ; i, free paired ventral spines ; pet., pectoral fin-spines ; plv., pelvic 

 fin-spines. 



parts are always preserved at the base of the pectoral fins, and 

 these seem to be merely dermal structures ; but their homologies 

 have not hitherto been satisfactorily determined. In Acan- 

 thodes (fig. 29) the pectoral spine (s) is supported by a hollow, 

 mesially constricted element (&) which abuts against the side 

 of its proximal end ; and at a short distance below this there 

 occurs a close series of short, fine dermal rays (r), sometimes 

 appearing as the fringe of a short, obtuse lobe, which may mark 

 the limit of the endoskeletal part of the appendage. In 

 Diplacanthus (fig. 30) there are two spines (s, m) in each 



