MORPHOLOGY OF THE COAL MEASURES AMPHIBIA. 29 



Coal Meastires. One large rib (plate 22, fig. 4) may represent a labyrinthodont, 

 but nothing is known of the species to which the rib belonged. 



(n) The pectoral girdle (187) is a very simple and uniform structure, although 

 the details of the association of the elements still remain to be determined. A 

 single, median, usually large and elongate interclavicle occupies the ventral line 

 of the chest. This is morphologically the same element which occurs in the middle 

 line of the chest of the lizards. It is a dermal bone and is usually, especially among 

 the Microsauria (462), highly sculptured. It varies considerably in size and shape, 

 but is remarkably uniform throughout the various groups. Lying anterior to the 

 interclavicle and overlapping its antero-lateral margins lie the two clavicles, which 

 are usually diamond-shaped and are sculptured, dermal bones. The position of the 

 coracoid is still uncertain, and in fact its clear association in the pectoral girdle of 

 these species is still a question. It seems to be constant in the European (186, 251) 

 species and is usually represented by a small rounded plate of bone, which in life no 

 doubt had a large amovmt of cartilage to form its borders. A cleithrum (285) has been 

 ascribed to one of the Linton, Ohio, species (plate 15, fiig. 3) by Jaekel (347), but this 

 needs confirmation. An osseous scapula is usually present, resembling the scapula of 

 modem salamanders, in that it was largely embedded in cartilage. The position 

 of the pectoral girdle is largely a matter of doubt, especially for the American spe- 

 cies. After death and before fossilization the girdle was always moved by post- 

 mortem shifting, so that its exact relation to the ribs and vertebral column is still 

 in doubt. Credner (186) has restored the pectoral girdle close behind the head, 

 which would cause an amount of rigidity in the body which probably did not exist. 



(o) The arm consists of the humerus, radius, ulna, and 4 digits. The characters 

 of the arm-bones are such as is constant among primitive animals and developing 

 mammals. The osseous portion is perichondral. Epiphyses are totally lacking and 

 it is doubtfvd if the endochondrium was at all ossified. The digits are often termi- 

 nated by ungual phalanges, although usually the terminal phalanx was merely 

 embedded in the web of the foot; and among the terrestrial forms a claw was well 

 developed. An osseous carpus is not known in the species from the Coal Measures. 

 Its impression indicates a broad hand, well adapted for swimming. 



{p) The pelvic girdle consists uniformly (462) of the ilium and ischium. A 

 small rovmded pubis is present in some of the later forms of Amphibia; it is, however, 

 totally absent from the Coal Measures species. The condition of the pelvis is 

 paralleled by the partially grown pelvis of mammalian embryos in which the elements 

 ossify in the order of ilium, ischium, and pubis. The ilium is always the larger of 

 the elements. It supported or was attached to the sacral rib by means of a liga- 

 mentous union. The ischium did not ossify completely until the animal was nearly 

 mature. The union between the elements of the pelvis was probably of a loose, 

 membranous sort or else the whole mass was embedded in cartilage; of the two 

 hypotheses the former is the more probable. 



The pubis is indicated as a calcified quadrangular plate in a specimen of Amphi- 

 bamus grandiceps Cope (478) from the Mazon Creek shales, and it is present as a 

 rounded osseous element among some of the Permian forms. 



