552 PEOF. T. W. BEIDGE ON THE MESIAL 



lamina of bone (PI. XXI. fig, 15, r.e.'^,p.s.), in the angle of which is 

 situated the " guard-spine," while the apex is suturally articulated 

 with the produced lateral margins of the first proximal segment 

 {r.e}, p s.). The third radial element (fig. 1-i, r.e.^) is more normal, 

 and consists of a proximal segment (p.s.) with a postero-superior 

 process and an ossified cubical distal segment (d.s.) embraced by 

 the cleft base of the third fin-ray. The proximal segment, like 

 those of the preceding radial elements, has the lateral margins of 

 its distal extremity produced outwards in the form of wing-like 

 laminae (fig. 15, r.e.^,p.s.), which, superiorly, form a transversely 

 elongated surface for the support of the defensive spine (fig. 14, 

 d.sp.), and at either extremity suturally articulate with the hinder 

 ends of the Y-shaped lamina of the second proximal segment 

 (fig. 15). The arraugement of the foramina for the transmission 

 of the erector muscles of the guard and defensive spines is very 

 similar to that described by McMurrich in the case of Amiurus 

 catus. In a dorsal view (fig. 15) it will be seen that the V-shaped 

 lamina, in conjunction with the lateral wings of the third proximal 

 segment, encloses a somewhat triangular space in which are situ- 

 ated the bases of the two spines and their supports. The large 

 foramen on each side of these structures (f^) transmits the 

 erector muscles of the defensive spine, the corresponding muscles 

 of the " guard-spine " passing from their origin to their insertion 

 through two much smaller lateral foramina {f^) which perforate 

 the distal end of the second proximal segment immediately 

 beneath the V-shaped lamina. 



There are eight fin-rays, which in order from before backwards 

 include (i.) the " guard-:^pi^e," (ii.) the large defensive spine, and 

 (iii.) a series of six soft multiarticulate rays. The third to the 

 eighth inclusive, that is the six soft rays, are perfectly normal in 

 their mode of support and in their relations to the last six of the 

 radial series. Each ray (fig. 14) is supported partly by the distal 

 segment of its proper radial element and partly also — and this is 

 more particularly the case with the third, fourth, and sixth rays — 

 by the articulation of its bifid cond} lar base with the distal ex- 

 tremity of the next succeeding proximal segment. The guard 

 and defensive spines, however, are somewhat peculiar. The 

 defensive spine, instead of being bifid, has a transversely elon- 

 gated base, divided into a median and two lateral condyles, and 

 apparently formed by the secondary fusion of the basal extremities 

 of an ordinary cleft ray. The lateral condyles articulate with 



