6 SHOULDER-GIRDLE AND BREAST-BONE. 



ON THE SHOULDEB-GIBDLE OP THE EISH-CLASS. 



As the lowest or Suctorial Fishes have neither limbs nor limb-girdles, I must begin with 

 the most generalised forms that possess these structures. It will be very convenient to consider 

 the endo-skeletal or essential parts first, where they are not only present in the greatest mass 

 and quantity, but also in that Order of Fishes in which there is no correlation of the exo- with 

 the endo-skeleton ; these are the Plagiostomes or Placoids ; the Rays and Sharks. Then I 

 intend to show how the internal skeleton of these parts is put into close relation with the large 

 " ganoid " plates that cover the body in certain types ; and which plates have to be described in 

 connection with the parts of the essential Shoulder-girdle : we have this correlation in the Sturgeon, 

 which has a soft endo-skeleton ; and in the Siluroid Callichthys, which has a hard one. 



Then we shall come to forms in which only the inner (subcutaneous or aponeurotic] parts 

 of these scales are, as a rule, developed in immediate contact with the true Shoulder-girdle. This 

 is the case in the ordinary (Teleostean) Fishes; but I shall pass to the typical through a 

 series of aberrant forms beginning with the most generalised of all, namely, the Lepidosiren. 

 There is no Sternum to be described in the Fish-class ; my work here, therefore, lies entirely with 

 the Shoulder-girdle. 



PISCES PLACOIDEI. 



A. No splint-bones ; Shoulder-girdle with large supra- scapulae entirely segmented off from 

 the scapulae. 



Example. Raia clavata, Linn. 



In Plate I, figs. 1 4, the Shoulder-girdle of the Thornback- skate (half-grown) is shown, of 

 the natural size ; whilst fig. 5 gives a view of a small part of the surface, magnified so as to show 

 the small, polygonal, bony plates that give strength to the cartilage. Before describing the 

 morphological characters of these parts, it may be said that these plates are not formed by ossi- 

 fication of the inner, soft layer of the tough, fibrous perichondrium, but small territories of the 

 surface-cells of the hyaline cartilage undergo immediate metamorphosis into bone : this I propose 

 to call " superficial endostosis ;" the other process, or that outside the cells, but taking place in 

 the inner part of the perichondrium, may be called " ectostosis." 



In one sense, this my first example is the most perfect Shoulder-girdle in all the Vertebrate 

 Sub-kingdom; but massive and complete as it is, being the foundation of two exorbitantly 

 large limbs, it is yet low and generalised in its morphological characters. At first sight it 

 appears to be an absolute girdle ; but upon closer inspection it is seen that the cervical vertebral 

 spine is closely wedged between the supra-scapulae (s. sc.), just as in the Bird the sacral vertebral 

 spine is wedged between the iliac crests, the hinder' counterparts of the supra-scapnlae. In that 



