STRUCTURE OP BOXE IX FISUES. 



83 



skeleton of a living or extinct species of fish, we can decide 

 whether or not it belongs to the Amioidei and Lepidosteoidei, 

 or to some other group. The histological structure of the bone 

 may therefore be of the greatest practical value for the identi- 

 fication of fragmentary specimens*. It also may pi^ove of gi^eat 

 importance in the interpretation of phylogeny. 



Text-fie-. IG. 



Enlarged view of a fragment of the vertebral centrum of Ophiopsis. 

 Lettering as in text-tia-. 13. 



We cannot as yet determine for certain which type of bone is 

 the more primitive ; but T am strongly inclined to believe that 

 the lepidosteoid type is the more specialised form, some of the 

 o.steoblasts having become modified to form tubules. This 

 conclusion is supported by the evidence of palaeontology, since no 

 Amioid or Lepicfesteoid is known below the Permian, while 

 Chondrosteans occur in the Devonian strata. In the absence of 

 decisive evidence we may suppose that the lepidosteoid structure 

 first appeared in the scales, then spread to the dermal bones, and, 

 finally, reached the deepest parts of the endoskeleton— this, of 

 course, is mere conjecture. At all events, since we find ordinary 

 bone in all the Osteolepidoti, Coelacanthini, Polypterini, and 



* The following is a list of the Actinopter3'gii examined :— Lepidosteoidei : 

 Lepidosteus, Aspidorhynclms. Amioidei : Eugnathidae— J^M^rwa^Atts, Caturus, 

 Heterolepidotus. Pachycormidae — Pachycormus. Semionotidse — LepidoUis, Dape- 

 dius. Macrosemiidae — Macrosemius, Ophiopsis. Pycnodontidae — Mesturus, 

 Gyrodus. AmndsR—Amia. Pliolidophoridje — PhoUdophorus. OligopleuridaB — 

 SpatJiiurus, Oligopleurns, CEnoscopus. Also Leptolepis, Thrissops, and a large 

 number of Teleosts. 



6* 



