742 General Notes. [September, 



Planorbes. The larger so-called species of Planorbis are divided 

 into two groups. First, those in which the whorls are rounded ; 

 that is to say if the tube or cone, as represented in the preceding 

 part of the paper, was cut transversely, the section would show a 

 rounded (not round) outline. Examples are the typical P.comeus 

 L. of Europe, P. guadaloupensis Sby., P. subcrenatus Carp., and P. 

 tumid us I'll-, of Nicaragua, a quite persistent form, not, however, 

 quite as rounded as the others. Second, those in which the 

 whorls are either planulate, angulated, carinated or sub-carinated, 

 which includes most of the larger North American species: 

 examples are P. corpulentns Sav, P. traskii Lea, P. occidentals 

 Carp, and P. bicarinatus Say. In these the tube, if cut trans- 

 versely, would present an outline more or less angulated. Forms 

 like P. trivolvis connect the two groups, for while in some in- 

 stances this species exhibits the rounded whorls of the first it 

 imperceptibly differentiates from the above to obtuse angulation, 

 and thence to the sub-connate forms of the second group. 

 Further remarks in illustration, with references to variations in 

 other species, follow. N 



Researches pN the Comparative Structure of the Cortex 

 Cerebri.— In the Philosophical Transactions of the Royal Soci- 

 ety of London for 1880, Dr. W. B. Lewis details the results of a 

 full investigation into the minute structure of the cerebral cortex 

 in the pig, with notes upon the histology of the same structure 

 in the sheep and cat, with a view of comparison between the 

 brains of these animals and that of the highest members of the 

 mammalian series. The general arrangement of the cells consti- 

 tuting the greater portion of the cortex of the brain of the pig, 

 is very similar to that found in the highest Mammals, and the 

 cortex of the sheep closely resembles that of the pig. Among 

 the chief facts of interest elicited are the following:' A five and 

 six laminated cortex is found in all, the fundamental structure of 

 the layers is similar, and divergence in type is induced through 

 the varied character and distribution of the units of these layers. 

 Variations in laminar type, whether in man or the lower animals, 

 center about the mid-region of the cortex ; and motor areas are 

 characterized by a five-laminated cortex and nested cells. In the 

 cat the cells of the third layer increase in size with their depth, and 

 the ganglionic cells are very large and crowded around the 

 crucial sulcus — this concentration is a feature of importance in 

 the Carnivora, and distinguishes them from the pig and sheep, in 

 which the ganglionic cells are widely spread and uniform ; and 

 from man and the apes, in which they are widely spread and w\n^ 

 in development. The ganglion cells of the sheep and pig differ 

 wholly in type from those of the higher mammals, and approach 

 closely in appearance the large pyramidal cells of the third layer 

 in man and the ape. 



