The homologue of the _ventral scutellæ_ is found in plesiosaurs, crocodiles, _Sphenodon_, and other reptiles in the "abdominal ribs," and the same myocommatous ossifications undoubtedly go to the formation of the chelonian plastron. What the causes were which produced the development of the ventral scutellæ to such a high degree among the primitive land vertebrates is uncertain, but they are certainly more highly developed among the primitive reptiles and amphibians than among the later members of those classes. Among the Amphibia of the Coal Measures they attained, in some forms, a high degree of development and differentiation. They are present in all families so far known, except the Tuditanidæ, in which the myocommata may have been cartilaginous. The Sauropleuridæ present the highest development of these structures among the American forms, in which the scutes are large and osseous. Among the Branchiosauria they are calcified or partially ossified and are always arranged _en chevron_ on the belly, chest, arms, and throat, their arrangement and direction of the _chevron_ being modified according to the myomeres of the various regions. The ventral scutellæ of the European Branchiosauria are figured and described fully by Credner (192, p. 21, figs. 4 to 11). [Illustration: Fig. 9.--Ventral scutellæ of _Micrerpeton caudatum_, a Coal Measures salamander from Mazon Creek. × 5. _f_, femur; _h_, humerus; _ls_, lines of scutes; _v_, vertebral column.] (_s_) _Scales_ (fig. 10 and plate 24, figs. 2 and 3) are present on the body of (462, 485) several species. It is a matter of regret that their preservation is so imperfect that nothing can be found out as to their structure. The Linton species, which possess scales, are, of course, carbonized and hence impracticable for microscopic study, and in the Mazon Creek species of _Amphibamus_ and _Micrerpeton_ the scales have been replaced by kaolin. The bodies of two species (_Cercariomorphus parvisquamis_ and _Ichthyerpeton squamosum_) of the Linton Coal Measures Amphibia were completely scaled. The scales in the Branchiosauria (462), so far as they have been observed, are slightly imbricated; rounded, with concentric markings after the manner of the modern cyprinoid fish-scale. They are extremely minute, and whether or not they covered the entire body of the animal is unknown. On the body of _Cercariomorphus_ the scales have the appearance of being tubercular without imbrication, and they apparently covered the entire bodily surface of the animal. [Illustration: Fig. 10.--Horny armor of back of _Hylonomus_. _a_, imbricated scales; _b_, horny plates; _c_, horny spines or tubercles; _d_, small imbricated scales. (After Dawson, based on a photograph.)]