528 DE. G. ELLIOT SMITH ON 



In Dasypus sexcinctus and Dasyptis villosus there is a sulcus wbich we may refer to as 

 simply a. This consists of a small slightly arched sulcus, about 10 mm. long, which 

 jmprints the pallium about midway between the corpus callosum and the superior 

 border in sucli a manner that the centre of the arc is placed above the prominent 

 splenium of the corpvis callosum. 



In Xenurus the sulcus a is much more extensive, and exhibits a jieculiar resemblance 

 to the fissure a in Ori/cteropiis, excepting that it lacks the posterior descending i^axi, 

 which we have called a' in Ilyrmecophaga and the Bradypodidse. 



It is not improbable, however, that this defect is more apparent than real. In 

 speaking of MyrmecojjJiaga we had occasion to point out how much lower the sulcus a 

 descends in this form than the corresponding part of the sulcus a does in Orycteropiis, 

 and we associated this with the fact that the sulcus a presents a constant relationship to 

 the rhinal fissure, which is placed higher up on the cerebral surface in the latter. The 

 rhinal fissure is placed much higher still in the Armadillos, and hence it is not 

 improbable that the apparently incomplete sulcus a in Xenurus may be analogous to the 

 whole of the extensive sulcus a in Orycteropus. 



In 3£anis, Pouchet * has figured an extensive longitudinal sulcvis analogous to the 

 sulcus a" in the Bradypodidse and Myrmecophaga. It is considerably longer than the 

 •corpus callosum, but quite simple. More recently Max Weber has given an excellent 

 figure of the mesial surface of the brain of Manis f , in which he represents under the 

 title "fssura sjdenialis " a longitudinal sulcus which extends the greater part of the 

 length of the mesial surface, and ends in front in a T-shape. In his figure there is a 

 faint line proceeding from the neighbourhood of the hippocampal fissure around the 

 posterior margin of the hemisphere on to the dorsal surface of this in a manner 

 similar to the sulcus u in Bradypus. There is, however, no mention of any such 

 sulcus in his description. Moreover, Ziehen, who has recently examined the brains 

 of four representatives of this genus, makes no mention of any such sulcus. He simply 

 states J that in Manis he found aJ/ssKva splenialis 15 mm. long, Avhich extended further 

 forward than the genu of the corpus callosum, and ended in a T-shape, 4*5 mm. from 

 the frontal pole. 



The specimen of Manis to which I had access in the College of Surgeons j)resented on 

 the mesial surface a sulcus exactly resembling that which Weber figures and describes 

 under the name " fssura splenutlis." 



Before passing on to the consideration of the cranial surface of the pallium we may 

 briefly consider, in the light of the imperfect data at our command, the significance of 

 this a-system of sulci, which in some form or other is present in all the heterogeneous 

 representatives of this order, with the exception of the smooth-brained Chlamydophorus 

 and Cycloturus. 



If we examine the mesial surface of the brain of the Sheep {Ocis aries) we shall find a 

 I'learly-defined series of sulci presenting an arrangement analogous to the a series in the 



* Pouehet, op. cit., torn. vi. pi. iv. fig. 10. t Max Weber, Zool. Ergebnisse, ii. tab. ix. lig. 69. 



t Ziehen, ' Das CcntraluerTCiis^vstem tier Monotremcn und Marsupialicr,' Jena, 1897. p. 156. 



