M. E. Bessels on Species of Aidix. 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 siu'face ; the outside has the same im- 

 pressed wi'inkles 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 sm*face 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 Saurocetes was 

 an animal of much smaller size. Supposing that the broken 

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

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

 that the whole lower jaw had ai^ extent of 30-32 inches ; 

 and in this case the whole skull may have been 38^0 inches 

 or 3^ feet long, prceter iwopter. If that is true, the whole ani- 

 mal (if it had the figui'e of a dolphin like Fontoporia) 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 figm'e of the lumbar vertebrae of Zeuglodon 

 that Saurocetes had an analogous configuration, its total length 

 may have been no more than 20 feet. 



VII. — Observations on tJie Species of Atax 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 aquaticus, whilst Kupffer subjected 

 the folded lamina (Faltetihlatt) discovered by Weissmann to a 

 thorough examination. Clapar^de promises us, in a memoir 

 hereafter to be mentioned, further contributions ; and quite 



* Translated by W. S. Dallas, F.L.S., from the ' Wiirttembergisclie 

 naturwissenscliaftliclie Jaliresliefte,' 1869, pp. 146-152, 



