THE ENDOSKELETOX 701 



The internal support of both shoulder and pelvic girdle consists of 

 inverted arches across the ventral side of the body, the limbs of the 

 arch extending dorsally. The part to which the limbs are attached is 

 called the girdle. The arch itself always forms in cartilage, though mem- 

 brane bone or dermal bone may be added. 



The typical girdle consists of three elementary parts, one dorsal and 

 two ventral, all of which meet at the point of attachment of the limbs, 

 and all contribute to form the socket, in the forelimb called the glenoid, 

 and in the hindlimb known as the acetabulum, and shows why we con- 

 sider pectoral and pelvic girdles and appendages homologues. 

 THE SHOULDER GIRDLE (Fig. 416) 



In fishes, the shoulder girdle is more or less U-shaped, with the 

 glenoid fossa at the dorsal end, and immediately dorsal to the fossa lies 

 the scapular region. Quite often, the dorsal part of the scapular region 

 is again divided, so that a supra-scapula is formed. In the skates the 

 supra-scapula articulates with the adjacent vertebrae; usually, however, 

 the entire girdle lies free from the axial skeleton. A pair of clavicles form 

 from the skin. These overlay the coracoid region of the girdle, and meet 

 in the midline, while a cleithrum, a second bone above the glenoid fossa, 

 forms. In some of the ganoids, it is the cleithrum which extends toward 

 the midline so as to take the strain ; in fact, it is assumed that in higher 

 groups, where the two halves of the cartilaginous girdle have separated, 

 the separation is due to the stress laid upon these parts. 



In the higher ganoids and in the teleosts, the cleithrum increases in 



garis (Toad) as examples of the arciferous type. 3, Adult, and 4, metamorphosing 

 Frog, Rana temporaria showing change from the arciferous into the firmisternal 

 type. 5, Hemisus guttatum (narrow-mouthed toad). 6, Breviceps gibbosus (tailless 

 amphibia). 7, Cacopus systoma (narrow-mouthed toad). Cartilaginous parts are 

 dotted. Ossified parts are white. Cl, clavicle ; Co, coracoid ; E, epicoracoidal 

 cartilage ; H, hiimerus, M, metasternum ; O, omosternum ; P, precoracoid ; Sc, 

 scapula; S.S, suprascapula. (From Gadow and Boulenger.) 



II. A, The skeleton of the pectoral fins and girdle of a dogfish, seen from the 

 ventral side. (After Borradaile.) cor., Coracoid region; gl., glenoid surface; h.r., 

 horny rays ; mpt., metapterygium ; mspt., mesopterygium ; ppt., propterygium ; 

 rad., cartilaginous rays ; sc., scapula. B. Ventral view of the shoulder-girdle 

 and sternum of a Lizard, Loemanctus longipes. (After Parker.) 1. Inter- 

 clavicle. 2. Clavicle. 3. Scapula. 4. Coracoid. 5. Precoracoidal process. 6. 

 Glenoid cavity. 7. Sternum. 8. Sternal bands not united. 9. Sternal rib. 

 C. Sternum and associated membrane bones of a Crocodile, C. palustris. (After 

 Shipley and MacBride. ) The last pair of abdominal ribs which are united with 

 the epipubes by a plate of cartilage have been omitted. 1. Interclavicle. 2. 

 Sternum. 3. Sternal rib. 4. Abdominal splint rib. 5. Sternal band. D. Lateral 

 view of the pelvis and sacrum of a Duck, Anas boschas (After Shipley and 

 MacBride.) 1. Ilium. 2. Ischium. 3. Pubis. 4. Pectineal process, the rudi- 

 ment of the prepubis corresponding to the pubis of the Lizard. 5. Acetabulum. 

 6. Ilio-ischiatic foramen. 7. Fused vertebrae. 8. Facet on which the projection 

 on the femur, the trochanter, plays. E, The breastbone and shoulder girdle of 

 a rabbit, seen from below and somewhat from in front. (After Borradaile). 

 acr., Acromion ; cl., clavicle ; cor., coracoid process ; cp., capitulum ; g.c., glenoid 

 cavity ; mb., manubrium ; mer., metacromion ; sc., scapula ; st.r., sternal portion 

 of a rib ; st. 2, st.6, second and sixth sternebrae ; tb., tuberculum ; v.r., vertebral 

 portion of a rib ; x., xiphisternum ; x.c., xiphoid cartilage. F. Sternum and sternal 

 ribs of a Dog, Canis familiaris. (After Shipley and MacBride.) 1. Presternum. 

 2. First sternebra of mesosternum. 3. Last sternebra of mesosternum. 4. 

 Xiphisternum. The flattened cartilaginous plate terminating the xiphisternum is 

 not shown. 5. First sternal rib. 



III. Pectoral girdles of two types of ganoids. A, Acipenser (Sturgeon) and 

 B, Polypterus (African ganoid), ct, cleithrum; cv, clavicula ; dr, dermal rays; g, 

 glenoid" surface ; r, cartilaginous radialia. (From Kingsley after Gegenbaur.) 



