VERTEBRA AND RIBS 



33 



The bony Ganoids (Crossopterygii and Holostei) have extensive 

 ossifications and distinct centra, these last, in larval Amia, being like 

 those of the adult sturgeon. Later a superficial ossification — espe- 

 cially on the sides between neural and haemal arches (fig. 20) results, 

 in the tail, in two rings, centrum and intercentrum, to each myotome, 

 the classical example of diplospondyly. In the trunk centrum and 

 intercentrum fuse (sometimes three of these rings unite and the 

 arch shifts to the intercalated ring). Amia and Polypterus have 

 amphicoelous vertebras; those of Lepidosteus (fig. 36) are opistho- 

 coelous. This is caused by an extension inwards of intervertebral car- 

 tilage cutting off the notochord, then an incision cuts the cartilage in 

 such a way that ball and socket result, the process resembling that 

 occurring in Amphibian vertebrae (p. 38). 



Fig. 36. — Lateral and anterior faces of Lepidosteus vertebra; (Balfour and Parker, 

 '82). bp, basal process; c, centrum; ic, intercalary cartilage; is, interspinous bones; /, 

 dorsal ligament; na, neural arch; r, hsemal rib. 



Amia and Lepidosteus have haemal ribs Kke those of Teleosts, but 

 the existing Crossopterygians {Polypterus and Calamoichthys) have, 

 in addition, pleural ribs articulated to parapophyses and extending 

 almost horizontally between ep- and hypaxial muscles (fig. 23). 



TELEOSTEI.— The vertebrae of Teleosts develop much hke 

 those of Holostei, caudal and cranial parts of two successive somites 

 fusing to a single centrum, which, with few exceptions, is amphicoel- 

 ous, enlargements of the notochord persisting in the cavities in the 

 centra, the cavities often connected by a dicentral canal filled with 

 the constricted chorda. In the early stages the parts are much as in 

 the lower fishes, and ossification starts from these and extends to the 

 centra. Frequently no ossification occurs beneath the arches, the 

 result (fig. 32, Z)) being very like the asterospondylous centra of 

 Elasmobranchs. Successive layers are added to the bone first 

 formed, these coming from the perichordal tissue without the inter- 

 vention of cartilage. Dorsal to the spinal cord there may be, as 



