316 ANATOMY OF VERTEBRATES. 



Having premised so much in reference to tlie Mammalian 

 skeleton generally or typically, its main modifications as exem- 

 plified in the several orders of the class, will next be noticed. 



§179. — Skeleton of Monotremata. — A. Vertebral Column. — The 

 principal osteological characters of this order are: — The extension 

 of the ' coracoid,' fig. 199, 52, o, as in Birds and Lizards, from the 

 scapula, 51, to the sternum, 5, and anchylosing at full growth with 

 the scapula, as at G, fig. 199 ; the epicoracoid, ib. n, as in Lizards ; 

 the marsupial bones, ib. x, x ; the supplementary tarsal bone, 

 ib. d, supj)orting the perforated spur, e, in the male ; the long 

 persistence of distinct pleurapophyses, j)l, in the vertebra dentata. 

 Both the genera have twenty-six ' true vertebrae,' of which 

 ^^Q seven are cervical ; but the Ornithorhynchus 



has seventeen and the Echidna sixteen dor- 

 sals, the lumbar vertebrie being three in the 

 latter, and reduced to the lacertian number 

 two in the Ornithorhynchus. 



The intervertebral substance is dense and 

 fibrous at its periphery, fig. 200, a, but the 

 fluid central part, h, fills a more definite 

 cavity in the Echidna than in higher Mam- 

 mals. ' 

 Lumbar rertei.ras, inter- Jn thc dorsal vcrtebrffi thc ucrvcs perforate 



A-ertebral cavities, Efliidua. '■ 



the neurapophyses ; but escape, as usual, at 

 their intervals in the cervical and lumbar regions. The dorso- 

 lumbar neural spines are short and subequal, fig. 201. The 

 ribs of the first six dorsals have ossified sternal portions which 

 articulate with the sternum ; in the succeeding vertebrae to the 

 fifteenth the sternal portions are cartilaginous, expanded, and 

 overlap each other, fig. 199 ; the last two pairs of ribs termi- 

 nate freely. Most of the vertebral ribs articulate over the in- 

 terspace of their own and the antecedent centrum ; a small 

 tubercle defines the neck of the rib, save in the last four ; but, 

 save in the first and second, does not articulate with the dia- 

 pophysis. The first dorsal pleurapophysis is broad, the others 

 are cylindrical and slender ; cartilage is interposed between the 

 bony pleur- and hajm-apophyses of the anterior dorsal vertebra, 

 as in the Crocodile. The sternum consists of four bones in 

 Ornithorhynchus, and of five in Echidna, The first, fig. 199, s, 

 is an unusually expanded ' manubrium,' receives the hasmapo- 

 physes of the first and second ribs, and supports a large T-shaped 



' 1.XXTX-. p. 375. 



