SKELETON. 41 
each scale truncate and this edge and the surface toothed. Cycloid and ctenoid 
scales intergrade and both kinds may occur on the same fish (many gobiids). 
AMPHIBIA.—A dermal skeleton occurs in the recent amphibians only as rows 
of plates in the cutaneous rings on the bodies of the cecilians and in the skin of the 
back of a few exotic toads. In some fossil stegocephalans there was a ventral 
armor and in others one protecting the whole body. The ventral exoskeleton, 
sometimes of scales or plates, sometimes long bars, is arranged in oblique rows, 
and is interesting as probably being the source of the gastralia found in many 
reptiles (infra). Episternum and clavicle were possibly dermal in these forms, 
but they will be described in connection with the shoulder ‘girdle. Apparently 
certain of the gastralia of these fossils were modified into comb-like organs which 
have been thought to have sexual significance. 
REPTILES.—The dermal skeleton is best developed in the turtles of living 
reptiles, though here it is closely associated with the endoskeleton. The dermal 
plates form a box for the protection of the body. This consists of a dorsal carapace 
and a ventral plastron, united to varying extents and each consisting of a number 
of elements. In the carapace there is a middle line of neural plates (fused with 
Fic. 33.—Section through developing vertebra, rib and exoskeleton of Chelone imbricata, 
after Gitte. ¢, cutis; cs, primitive vertebral body, ef, epidermis; m, external oblique muscle; 
p, peri ichondrium; r, rib; sp, spinal process. 
the vertebre), marginal plates around the margin, and costal plates, fused to 
the ribs, between neurals and marginals. The plastron (fig. 34) usually consists 
of nine plates, wholly dermal, the names shown in the figure. ‘The three posterior 
pairs are regarded as the same as the gastralia of other reptiles, the anterior pair as 
the clavicles, while the unpaired entoplastron is supposed to be homologous with 
the episternum of other tetrapoda. 
Some of the extinct crocodilia were armored with closely applied scales and 
these have been retained in the existing species in a reduced condition. They 
also have well developed gastralia. These are of rods dermal bone in the ventral 
body wall between the true ribs and the pelvis, and so closely resemble ribs that 
they were called ‘abdominal ribs.’ They do not meet in the middle line; each, 
except the first, consists of two distinct parts, and the pairs correspond to the 
somites in number. In Sphenodon (fig. 35) the gastralia are more numerous than 
the somites. 
In a few lizards there are dermal scales, while the extinct stegosaurs had 
