126 



ORDERS OF FISHES. 



tological point of view, however, the most important group 

 of the Malacopteri is that of the Sheat-fishes {Siluridm). 

 The importance of this group does not arise from the 

 occurrence of many fossil fishes which can be definitely 

 referred to it, but from its relationship, on the one hand, 

 to the " Saurodont " fishes of the Cretaceous, and, on 

 the other hand, to the much more ancient group of the 

 " Placoderms " of the Silurian and Devonian, The living 

 forms are cliiefly noticeable because they are amongst the 

 small number of living fishes possessed of structures of the 

 same nature as the fossil spines known as " ichthyodoru- 

 lites." The structure in question consists of the first ray 

 of the pectoral fins, which is largely developed and consti- 

 tutes a formidable spine, which the animal can erect and 

 depress at pleasure. Unlike the old " ichthyodorulites," 

 however, the spines of the Siluridce have their bases modi- 

 fied for articulation with another bone, and they are not 

 simply hollow and implanted in the flesh. The head is 

 protected by an exoskeleton of dermal bones, thus coming 

 to resemble structures witli which the palaeontologist is 

 familiar in the head-shields of the extraordinary Pteraspis, 

 Pterichthys, Coccostcus, and other Paleozoic Placoderms. 

 The latter appear, however, to be truly Ganoids ; but we 

 may more safely place in the neighbourhood of the " Silu- 

 roids " the large and important Cretaceous family of the 



Fig. 495.— Skull and fore-part of tlie skeleton of Portlmis, restored (after Cope). Cretaceous. 



" Saurodont " fishes, typified by such genera as Porthcus (fig. 

 495), Ichthyodcctes, Baptinus, Hypsodon, Saxiroceplialus, and 

 various allied forms. The fishes of this group were all car- 



