10 SHOULDER-GIRDLE AND BREAST-BONE. 



I may remark, before passing on to the Ganoids, that the abortive development of the 

 supra-scapula in Squatina is very interesting ; for this part is altogether indistinguishable in the 

 Lepidosiren r and in the Teleostei ; the absence of a transverse cleft between the supra- 

 scapula and the scapula in Galeus is a morphological character so constant in the Vertebrata, 

 generally, that the Rays and Sturgeons are, as far as I know, the only Families which are 

 exceptional in this respect. J shall show that even the Frogs and Toads agree, in this part of 

 their Shoulder-girdle, with the Dog-fish (Galeus), and not with the Ray and the Sturgeon. It is 

 evident that, in using morphological " cleavage " as a measure of height in the vertebrate scale, we 

 must be very cautious how we apply the rule, and not forget what striking exceptions there are 

 which, in some considerable degree, diminish its value. 



PISCES GANOIDEI (A., Recent forms) . 



A. Large dermal (" ganoid") plates covering the head and various parts of the body, many 

 of which are correlated to the feebly ossified endo-skeleton as splint-bones ; true Shoulder-girdle 

 entirely unossified. 



Example. Acipenser sturio, Linn. 



If the anatomical student has well considered the perfect independence of the endo- and 

 exo-skeleton in the Placoid Fishes, he will be in a state of mind fit to receive the intelli- 

 gence which the study of the Sturgeon will impart to him. Here the endo-skeleton is largely 

 developed, but for the most part unossified, and arrested as to type ; the ossification also, when 

 it does occur, as on most of the cylindrical cartilages, is as a sheathing " ectosteal" layer, which 

 invests the cartilage for a long time before setting up "endostosis" within; this is in harmony 

 with what is found in the embryos of all the Teleostei, and in the adult Fish in some instances, 

 e.g. Gobius minutus, Linn. But in the region of the body treated of in this Memoir we find no 

 ossification whatever ; and therefore the relation of the exo- to the endo-skeleton is very easy of 

 interpretation. 



Here commences a remarkable organic affinity of the " bars of the skin " for the bars of the 

 flesh ; the former, in certain regions, cleaving to the latter as the bark of a tree does to the 

 wood, or as the ivy does to the trunk that supports it ; and, like the ivy, the splint-system seems to 

 injure what it protects, for in many instances there is an evident abortion of the endo-skeleton as 

 a correlate of the overgrowth of the outer bony plates, as may be seen in the trunk of the 

 Tortoise. There the affinity of the outer for the inner skeleton is intense ; here, in the Sturgeon, 

 it is less pronounced ; ultimately, in the warm-blooded classes, we shall see that parts of the 

 exo-skeleton do actually graft themselves upon parts of the endo-skeleton, as the Mistletoe grafts 

 itself upon a dicotyledonous tree. 



But there is nothing loose or accidental in the manner in which the strong " ganoid" plates 

 of the Sturgeon apply themselves to the soft inner parts ; and having once determined the true 

 homologies of those endo-skeletal regions, we can also be sure of their dermal correlates in Order 

 after Order and Class after Class, until we reach even our own species; the genesis of the 



