110 STUDIES ON THE ELASMOBRANCH SKELETON, 



interspace, the pointed dorsal extremity of each half of the arch 

 being received into a socket formed for it by a produced portion of 

 the spinous ridge of the anterior vertebral plate in such a way that 

 motion is only possible round a transverse axis passing through the 

 two joints. The outer border of the transversely expanded lateral 

 portion of the girdle gives origin behind nearly at right angles to 

 an exti*emely prominent lamella, concave inwards and convex out- 

 wards and directed outwards as well as backwards. This lamella pre- 

 sents externally four horizontal ridges, the most prominent being 

 the third reckoning from above downwards. This ridge connects 

 the articular surfaces for the pro- meso- and metapterygia, and 

 between the two last itself gives attachment directly to a number 

 of fin-rays. The articular surface for the propterygium is placed on 

 the produced border by which the expanded part of the hoop and 

 the lateral lamellae unite. A little behind it is the less prominent 

 articular surface for the mesopterygium. Between the two are two 

 large nerve foramina, the one above and the other below ; these pass 

 directly from within outwards. The articular surface for the 

 metapterygium is placed on the posterior border of the lateral plate 

 at the extremity of the prominent ridge ; it is a little less conspi- 

 cuous than that for the propterygium. About midway between the 

 ridge which bears the articular surfaces and the one above it is a 

 horizontal row of fine small pores. The articular surface for the 

 branchial arch is an ovate elevation situate close to the inner 

 border of the lateral portion of the arch. The ventral bar is 

 deeply grooved below. Among described forms the shoulder-girdle 

 of Trygonorhina approximates most nearly to that of Rhino- 

 batus (Gegenbaur, 1. c. II., p. 82, pi. V., fig. LA.) 



The propterygium consists of a stout proximal portion with four 

 short distal joints, with the extremity of the last of which three 

 irregular rays articulate. The mesopterygium is small ; between it 

 and the metapterygium is a wide interval where the fin rays 

 articulate directly with the shoulder-girdle itself. The metapteiy- 

 gium is smaller than the propterygium ; two short cartilages are 

 added to its extremity. The majority of the fin-rays of the pectoral 

 fin bifurcate at their extremity. 



