120 THE NAUTILUS. 



Mutelid genera Glabaris, Fossula, Monocondylcea, and further, the 

 South American Hyriina : Hyria, Tetraplodon and Diplodon, group 

 together, and their differences from the North American and Asiatic 

 forms discussed above are such that we are fully justified in placing 

 them in a separate family, whatever the name of the latter may be. 



3. Within this Afro-American family we may distinguish two 

 types, differing in very important characters, which might properly 

 be subfamilies (Hyriirue and Mutelina). 



4. The systematic affinity of African and South American genera, 

 already pointed out by v. Ihering, is much more striking, and much 

 better supported, than before, and it is of the most important value 

 for zoogeographical questions, namely, for the theory of an old con- 

 nection between these continents (Archhelensis theory of v. Ihering). 



5. The natural system of the Naiades expresses a tendency of 

 development and specialization of three chief anatomical characters, 

 which in turn are connected with certain functions. These are: (a) 

 the separation of the original simple branchial chamber into two 

 chambers, branchial and suprabranchial, by the diaphragm; (b) the 

 restriction of the branchial and anal openings of the mantle edge to 

 defined parts of the latter, with a tendency to form siphons; (c) the 

 development of the gills into organs for carrying the eggs and larvae 

 (marsupium), and the specialization and adaptation of the gill struct- 

 ure for this purpose. 



(a) In the development of the diaphragm three types are distin- 

 guishable. The most primitive stage is represented in Margarita-no,, 

 where the diaphragm is formed by the growing together of the inner 

 laminae of the inner gills of the two sides of the body, and the fusion 

 of the outer laminae of the outer gills with the mantle. But here the 

 diaphragm is yet incomplete, in so far as the outer lamina? of the 

 outer gills remain free at the posterior end, and are not connected 

 with the mantle to its posterior margin. 



(7(9 be concluded.) 



NOTES. 



Dr. W. H. Dall was elected president of the American Palaeon- 

 tological Society at its recent Pittsburg meeting. 



