M. E. Bessels on Species ©/"Atax. 55 



on the upperside more than half an inch, and on the hinder 

 end less than a quarter of an inch. The interior is entirely 

 open, with a smooth sm-face ; the outside has the same im- 

 pressed wrinkles as the anchylosed part of the jaw on the exte- 

 rior sm-face, but much smaller wrinkles on the interior, where 

 the two branches are united to each other. Here the structure 

 of the surface is finer, and the bone more delicate. As a part 

 of this surface is broken off, I cannot ascertain the extent 

 of the opening of the alveolar channel, which was on this 

 side of the articular branch. The only particular character 

 which I see here is the presence of a sharp edge on the lower 

 border of the branch, beginning a little behind the alveole of 

 the last tooth, and increasing in elevation behind. 



Finally, comparing the known part of the animal with the 

 lower jaw of Zeuglodon^ there is no doubt that Saiwocetes was 

 an animal of much smaller size. Supposing that the broken 

 tip of the lower jaw was 7-8 inches long, and the wanting 

 end of the articular branches 5-6 inches, we may presume 

 that the whole low^' jaw had an extent of 30-32 inches ; 

 and in this case the whole skull may have been 38-40 inches 

 or 3^ feet \on^, prceter propter. If that is true, the whole ani- 

 mal (if it had the figure of a dolphin like Pontoporid) may have 

 been 15-16 feet long, as we know from my description that the 

 skull occupies one fifth part of the entire body ; or if we judge 

 from the elongated figure of the lumbar vertebrge of Zeuglodon 

 that Saurocetes had an analogous configuration, its total length 

 may have been no more than 20 feet. 



VII. — Observations on the Species of Aisiis. parasitic upon our 

 Freshwater Mussels. By Emil Bessels*. 



It is comparatively but a short time since the embryology of 

 the Arthropoda received far less attention than this interesting 

 branch of science really deserved ; and yet, since the classical 

 memoir of Weissmann upon the development of the Diptera, 

 it may almost be said to have become a favourite study with 

 zoologists. In the course of the last few years there have ap- 

 peared a series of works upon this subject, such as Mecznikow's 

 embryological studies on insects and Dohrn's on the embryonic 

 development of Asellus aquaticusj whilst KupfFer subjected 

 the folded lamina [Faltenhlatt) discovered by Weissmann to a 

 thorough examination. Claparfede promises us, in a memoir 

 hereafter to be mentioned, further contributions ; and quite 



* Translated by W. S. Dallas, F.L.S., fron* the ' Wiirttembergische 

 naturwissenschaftliche Jahreshefte,' 1869, pp. 146-152. 



