92 IIISTIONOTUS ANGULARIS. 



are on the side of the head in the Shark, and on the lower surface 

 of the head in the Ray. The skull and lower jaw are well 

 developed, but there are no cranial bones ; the vertebral column is 

 sometimes composed of distinct vertebrae, and sometimes they are 

 cartilaginous; the pectoral arch has no clavicle. Most of the 

 Plagiostomes have the lower lobe of the tail, which is slightly turned 

 upwards, larger than the upper, and is termed heterocercal ; but this 

 is not always the case. In several of the Rays the tail is straight 

 and the fin equally developed above and below, dipliy cereal. In 

 consequence of the perishable nature of their skeletons only detached 

 spines, teeth, &c., are usually met with in the rocks ; however, 

 complete fish have been met with in the Lias of Lyme Regis. The 

 P/agiostomes have been found in the Upper Ludlow rocks at the 

 close of the Upper Silurian. Their early forms were provided 

 with large fin-spines, known to geologists as ichthyodorulites. The 

 Rays appeared before the Sharks, occurring as early as the 

 Carboniferous, continuing through the Secondary and Tertiary 

 Rocks. 



At the present day the skeletons of ganoid-fish show great 

 variations from a condition of primordial cartilage to complete 

 ossification, some having no greater development than is exhibited 

 in the persistent cartilage of the Ray and Shark. Others 

 approach the Teleosteans so closely that their ganoid character 

 can only be identified by other distinctions ; the vertebral 

 column, with few exceptions, remain cartilaginous throughout life, 

 and the skull, excepting among the older members of the order, has 

 distinct cranial bones, and is furnished with a lower jaw. The body 

 has an outer covering of rhomboidal shining ganoid scales, composed 

 of two layers, the inferior bone, and the upper enamel ; they are 

 placed edge to edge in oblique rows. In the older forms there are 

 no vertebral centra, and the skull, which is cartilaginous, is 

 protected by plates, whereas in the later forms the margins of the 

 vertebrae are either ossified or have osseous rings enclosing the 

 primitive matter of the notochord. Some, however, are similar to 

 the Tcleoateans. The gills are free and protected by an operculum. 



