120 Transactions. — Zoo lotjy . 



so, those of each side unite by their ventral ends, forming a 

 longitudinal band of cartilage (PI. XIX., fig. 5, st.) ; the two 

 bands thus constituted then approach one another and unite 

 in the middle ventral line (tig. 6), foi-ming a median structure ; 

 areas of connective tissue appear separating it from the 

 ventral ends of the ribs, and thus the sternum is formed as 

 a purely costal product. 



In the tailed Ampliibia the sternum has a double origin : 

 A pair of cartilaginous bands appear in the inscriptiones 

 tendiueae of the mm. recti abdominis, and a pair of narrow strips 

 are separated off from the posterior borders of the coracoids. 

 From these four elements the sternum is produced. Euge con- 

 siders that the fu'st-named chondi'ites'" are to be looked upon 

 as vestigial ribs ; the others are obviously parts of the shoulder- 

 girdle. 



In the tailless Amphibia, such as the frog, the sternum has 

 a similar origin, but in many genera there are formed in addi- 

 tion paired cartilages in front (cephalad) of the shoulder-girdle, 

 and arising by the detachment of narrow bauds from the pro- 

 coracoids. These unite in the middle line, and form a median 

 cartilage (fig. 3, pr. om. st.), which was distinguished by my 

 father as the omostermim, the name sternum being restricted 

 to the post-coracoidean element (jJt. om. st.). As, however, 

 both structures are formed from the shoulder-girdle, Albrecht's 

 suggestion to call the anterior aoxiWdigQ ih.e pre-omostcrmun, 

 and the posterior the post-omostcrnum, is worthy of adoption, 

 especially as these names suggest homologies with the 

 similarly-formed median elements of the pelvis, the pre-pelvi- 

 stenium (epipubis) eaidi post-pelvisternum (os cloacae). 



In order to form any clear notion of the origin of this 

 element, of the skeleton, a sternum in some form lovrer than 

 the Amphibia is obviously a desideratum. I beheve I have 

 discovered such a structure in the Perlon Shark (Notidaniis 

 in(liciis).\ 



The shoulder-girdle of Elasmobranchs is an inverted arch 

 of cartilage attached to the vertebral column either by muscles 

 only (Selachians), or by fibrous union (Kays), and having the 

 pectoral fins attached, one on either side, to its posterior 



• Chonclrite — an independent cartilaginous element or centre of 

 chondrification. 



t Since writing this paper I find that the structure called in the 

 ensuing description the pre-omosternum was discovered in 1884 by 

 Haswell, who says, "The shoulder-girdle "of Notidamis indicJis] is re- 

 markable for the presence in the middle ventral luie of a distinct four- 

 sided lozenge-shaped cartilage, let into the arch, as it were, in front. 

 The intercepted cartilage is temptingly like a pre-sternal, 

 but the absence of such an element in the skeleton of any group nearer 

 than the Amphibia seems to preclude this explanation " (Proc. Linn. Soc. 

 N.S.W., vol. ix.).— March, 1891. 



