1906,] ON THE SKULL OF A HORSE SHOWING PREORBITAL PITS. 377 



Tiliqua scincoides, in which species I have recorded * the structure 

 in question as I believe for the first time ; for the elements are 

 larger and more thoroughly chondrified, and thus more easily 

 distinguishable from the tendinous intersections of the abdominal 

 muscles in which they lie than in Tiliqua, as will be seen in the 

 figure (text-fig. 84). When the outermost layer of the abdominal 

 musculature is raised from the deeper layer, the abdominal rilDS 

 are raised ^ with the former and can thus be seen to overlie 

 the true ribs which occur in the deeper layer of the ventral 

 musculature. 



Three pairs of abdominal ribs meet in the middle line and thus 

 form a series of three chevrons. The first two of these possess a 

 forwardly-directed process of the triangular plate which forms the 

 region where the two ribs of the pair are fused. Behind these 

 comes one pair of abdominal ribs, which does not — but only just 

 does not— meet in the middle line. A fifth and sixth rudimentary 

 pair exists ; there is a true rudiment on the right side of a 

 seventh abdominal rib. Behind this only the tendinous inter- 

 sections of the abdominal muscles are visible. In the region 

 of the parasternum the true ribs do not reach the middle line as 

 cartilaginous rods, and, as already mentioned, they are overlapped by 

 the gristly rods of the parasternum. As Prof. Parker has pointed 

 out t, there are five pairs of true ribs attached to the sternum in 

 TrachysauTus. He does not, however, mention that a pair behind 

 these also meet and fuse in the middle line a little way behind the 

 sternum. These true ribs meet and fuse superficially and exactly 

 resemble the succeeding abdominal ribs, so far as the median region 

 is concerned. This, however, can invalidate no homology, for the 

 exposure of a true additional piece of xiphisternum is simply due 

 to the absence of pectoral muscles ; and in any ca,se the remaining 

 pieces of cartilage so entirely overlap so considerable a portion of 

 the ^ true ribs that they cannot possibly be regarded as the 

 equivalent of their median ventral extremities, which, indeed, 

 themselves reach to within a millimetre or two of the ventral 

 middle line. 



Mr. R. I. Pocock, F.Z.S., exhibited the skull of a Horse to show 

 the preorbital pit, and made some i-emarks upon the occurrence of 

 this feature in the skulls of extinct and existing Equida;, and 

 commented on its supposed homology to the prajorbital pit of 

 Hijpyarion and upon the systematic value that has been attached 

 to it. 



The following papers were read : — 



* " On the Presence of Parasternum in . . . Tiliqua, &c.," P. Z. S. 1904, vol. ii. 

 p. 154. 



t Monograph on Shoulder-Girdle, Ray Soc. 1868 p. 114. 



