308 DK- G. ELLIOT SMITH ON 



dorsal commissure is represented as an obliquely-placed, inverted, U-sliaped structure, 

 with symmetrical limbs of approximately the same shape and size. It seems probable, 

 however, that there is a very definite attenuation and bending of the upper part of the 

 psalterium in both species of Dusyjnis. 



In Tolypeutes tricinctus. Garrod * represents a dorsal commissure of the same shape 

 as that represented by Turner in Dasy^ous, but with a corpus callosum slightly larger 

 than the psalterium. The specimen of Tolypeutes at my disposal was not sufficiently 

 well-preserved to permit me to supplement Garrod's brief notes. 



The commissures in the brain of Xeimrus unicinctus have not hitherto been described 

 or figured, so far as I am aware. In the well-preserved specimen representing this 

 genus in the College of Surgeons, the dorsal commissure very closely resembles the 

 appearance figured by Turner in Dasypus f . 



We can say that in the three genera Dasypus, Tolypeutes, and Xeimrus, the corpus 

 callosum is very small and pointed, obliquely situated, and of apj)roximately the same 

 size as the jssalterium. 



In the specimen of Tatusia in the College of Surgeons, the psalterium presents features 

 similar to those which we have described in Dasypus villosm, but the corpus callosum is 

 considerably larger than the psalterium, and is plumpei', longer, and of more unifoi'm 

 thickness than the corpus callosum of the other Armadillos. In this specimen the dorsal 

 commissru'e as a whole w^as not unlike that found in the Hedgehog {Erinaceus) and in 

 the Manidfc. Such being the case, it is surprising to find the following observations of 

 Poucliet % : — " Sur la coupe du cerveau dc I'Encoubert [Dasyj^iis] le corps cfilleux mesure 

 6 millimetres de long et 1 millimetre d'epaisseur; c'est chez un jeune Cachicame 

 [Tatusia] qu'il nous a montre les proportions les plus exigues. Nous I'avons vu aussi, 

 sur cet animal, nettement recevoir des fibres de la partie anterieure et de la partie 

 posterieure de I'hemisphere qui viennent s'y jeter en longeant la scissure mediane." 



It is probable that the immediate ancestors of the Dasypodida' possessed a considerably 

 larger corpus callosum than the existing types, for otherwise it would be difficult to 

 understand the stretching of the psalterium. If this be so, the corpus callosum has 

 undergone a retrogressive diminution in size, and the condition in my specimen of 

 Tatusia, rather than that of the other Armadillos, would be nearer the j)rimitive one. 



In Chlamydopjhorits we find a very peculiar dorsal commissure conforming to the 

 same type as Dasyims villosus, but in which the retrogressive changes resulting in the 

 diminution of the corpus callosum have gone much further. We find a large psalterium 

 divided in a typical manner into dorsal and ventral portions, and a cor2)us callosum so 

 reduced in size that it is only slightly larger than the psalterium dorsale. This is one 

 of the most extreme forms of reduction of the corpus callosum met with in the Eutheria. 

 If this process of reduction were to be carried much further in Clilamydopliorus we should 

 reach a state of cerebral modification wdiich we find in the Marsupial Notoryctes. 



* Garrod, op. cit., Proc. ZooL Soc. Loudon, 1S7S. 



t Unfortunately, in tlie jirocess of mounting, a glass rod bad l.ieen jushcd through the junction of the psalterium 

 and corpus callosum, thus rendeiing imjitssible an acC'urate description of this region. 

 J Pouchet, op. cit. torn. vi. p. 309. 



