TBANSACTIONS OF SECTION D. — DEPT. ANATOMY AND THYSIOLOGY. 599 



The author had made a considerable number of observations in different genera 

 of Simiae and Prosimiae, from which it appeared that in the whole of the American 

 monkeys examined without exception, amounting to 40 specimens from the genera 

 Mycetes, Ateles, Oebus, Pithecia, Nyctipithecus, Callithrix, and Hapale, the union 

 at the ptereon was spheno-parietal ; and the same was found to be the case in the 

 18 skulls observed of Lemur, Galago, Loris, and Oheiromys. 



In 96 skulls observed belonging to monkeys of the Old World, considerable variety 

 was found. In 23 of Macacus, 1 Colobus, and 8 Cercocebus, all had squamo-frontal 

 union ; of 33 baboons, the same kind of union existed in 29, in 1 double and in 1 

 xmilateral spheno-parietal union was observed, and in 3 cases epipteric bones were 

 present. In 10 skulls of Semnopithecus observed, 9 had spheno-parietal and one 

 squamo-frontal union ; and in 20 skulls of Cercopithecus there was spheno-parietal 

 union in 10 and squaino-frontal in 7 on both sides, and unilateral union of the two 

 different kinds in 3. 



The author left for future consideration the causes determining these varieties, 

 more especially as connected with the origin and mode of ossification of the 

 epipteric bone ; but he showed the probability of a number of the varieties being 

 attributable to the existence of such an intermediate bone at an early period, and to 

 its union with one or other of the bones surrounding the ptereon. This union 

 might be with alisphenoid, parietal, or squamous, but very rarely with the frontal 

 bone. The author considers it doubtful that the epipteric is homologous with the 

 posterior frontal, first described by Serres, afterwards by Rambaut and Renault, 

 and more recently by Von Ihering (1872), but believed that the further explanation 

 of the differences shown to exist in the mode of union of the bones at the ptereon 

 is to be sought for with success in the histoiy of their development, upon which he 

 expected some light to be thrown by observations in which he was engaged. 



Dr. Thomson also directed the attention of the section to some peculiarities in 

 the mode of ossification of the bones of the trunk in the young gorilla, which he 

 connected with the very frequent varieties observed in the adult condition of these 

 and other anthropoid apes. 



These observations referred — 1st, to the condition of the sternum as regards the 

 relative size of the presternum and mesosternum, and the number and place of im- 

 plantation of the costal cartilages into the sternum; 2nd, to the number of vertebral 

 ribs, and the occasional development of additional ribs upon the lumbar vertebrae ; 

 and 3rd, to the variations in the relative number and state of development of the 

 lumbar, sacral, and coccygeal vertebras in the four genera of anthropoid apes. 



The author reserved to another occasion the fuller description of these observa- 

 tions, which were still incomplete. 



2. The Intrinsic Muscles of the Mammalian Foot. By D. J. Cunningham, 

 M.D., F.E.8.E., Ser.ior Demonstrator of Anatomy, University of Edin- 

 burgh. 



The typical arrangement of the intrinsic muscles of the pes is the same as in the 

 manus, and this arrangement is seen to best advantage in the feet of certain of the 

 marsupialia. In these animals the muscles are laid down in three layers, viz. : — 



(1) A plantar layer of adductors. 



(2) A dorsal layer of abductors. 



(3) An intermediate layer of flexores breves. 



According to this disposition each digit is furnished with three muscles — one 

 from each layer. 



Deviations from the typical arrangement may take place by suppression or 

 fusion of certain of the elements of the different layers. Fusion of the constituents 

 of the intermediate and dorsal layers is extremely common, whilst fusion of the 

 intermediate and plantar muscles is a very rare occurrence. 



But this disposition of the intrinsic foot muscles does not account for the pre- 

 sence of an opponens muscle. This muscle may proceed from one of two sources. 

 Most commonly it is a development from the flexor brevis, and is thus associated 



