extrascapular series 



posttemporal 



I posttemporal 



extrascapulars n~\/ 



extrascapulars 



posttemporal 



'i I cleithrum 

 clavicle 



interclavicle (dashed) 



D CORNUBONISCUS 



ACIPENSER 



LEPISOSTEUS 



AMIA 



H 



SALMO 



Figure 6-63. Pectoral girdle of octinopterygian fishes. A, B, C, lateral, dorsal, and ventral view 

 of Po/ypferus; D, lateral view of Cornuboniscus budeusis, a palaeoniscoid; E, Acipenser fu/vescens; F, 

 tepisosfeus; G, Amio; H, So/mo. (A, B, C after Jarvik, 1944b; D, after Wfiite, 1939) 



chimaerids has been cited as an important liniiing feature. 

 The presence of an additional anterior clasper in the 

 chimaerid and a clasper on the head, viewed along with the 

 variety of structure of the type of clasper observed in sharks, 

 suggests parallel development rather than common ancestry. 



Other fishes 



The Acanthodii, arthrodires, and agnaths had fins that 

 are of particular interest since they appeared to be parallel- 

 isms to those already discussed. The pectoral girdle of 

 acanthodians has a scapular ossification above the pectoral 

 spine and a coracoid ossification below it (Figure 5-21). The 

 fin has a small basal lobe; there is a large spine in front and 

 behind this a membrane (Figure 6-68). In the base of this 

 fin, a few cartilaginous elements have been observed. The 

 pattern of these elements is only poorly known but suggests 

 a concentrated condition, with the elements radiating back 

 and outward from the base of the spine. The acanthodians 



are also of particular interest because there were more than 

 two pairs of spines in some of them. This multiplicity of ap- 

 pendages is unique (Figure 2-15). 



The arthrodires had an extensive trunk armor, in part 

 corresponding to the dermal pectoral girdle of other fishes 

 (Figures 6-69, 6-70). There is a posttemporal, supraclei- 

 thrum, postcleithrum, cleithrum, clavicle, and interclavicle 

 (paired or single), as well as a dorsal medial plate, postven- 

 trolateral plates, spinal, ventral cleithral, and medial ven- 

 tral plates. This trunk armor is much reduced in some types, 

 extensive in others. Inside of this armor is a cartilaginous 

 scapulocoracoid extending ventromedially, as a clavicular 

 process, to contact or join its opposite across the midline. 

 There is no particular scapular process and when pectoral 

 spines are present there is a cartilaginous core (see Stensio, 

 1959). 



The direction of evolution in the group has been difficult 

 to decide upon. Stensio suggested that the primitive condi- 

 tion was one with a broad-based fin. This type could follow 



184 • THE VERTEBRATE BODY SKELETON 



