84 NATURAL SCIENCE. Feb., 



Society (1895, PP- 2 4 I_ 5° > pl s< xi.-xiv.) on some of those slug-like 

 molluscs which, after the manner of our own little Vitrina, are too 

 big for their shells. The specimens in question were collected in 

 Borneo by Mr. Everett, who has added so largely to our knowledge 

 of the Mollusca inhabiting that interesting region. The conclusions 

 to which the authors came are, as might almost be expected, that 

 these slug-like Bornean forms bear the same close relationship to the 

 shell-bearing molluscs, among which they are now found living, as do 

 similar forms in other quarters of the globe, and that future research 

 will clearly show that many of the slugs cannot rightly be placed in 

 families by themselves, but will find their true position before or after 

 the genera they have developed into or are descended from. 



The Aptychus. 

 So much has been written on the structure and functions of the 

 curious body or bodies known as the Aptychus, which are shaped 

 something like the two valves of a Trigonia shell, and are found in 

 connection with ammonites, that a paper recently published by Dr. 

 Richard Michael, in the Zeitschrift der Deutschen geologischen Gesellschaft 

 (Jahrg., 1894, pp. 697-702, plate liv.; 1895) is'of general interest. -Dr. 

 Michael has discovered in the museum of Breslau University a slab 

 of Solenhofen slate with a specimen of Oppelia steraspis, the body- 

 chamber of which is not merely closed by an aptychus, but contains 

 some sixty tiny shells of young individuals, each with its own little 

 aptychus. The conclusions drawn from this discovery are that the 

 young of the ammonites were carried for some time in the shell of the 

 mother, just as they are in the nidamental shell of the modern 

 Argonauta, that the aptychus and the shell were both developed at a 

 very early stage, that an aptychus was possessed by all the individuals 

 of a brood, and therefore that the aptychus cannot be considered as a 

 structure confined to the female and intended to protect the nidamental 

 glands. As to the function of the aptychus, Dr. Michael endorses the 

 opinion now generally held, that it was a true operculum to the shell, 

 covering the mouth of the body-chamber when the animal was with- 

 drawn into it. 



The Embryology of Cirripedes and Isopods. 



In the last few years a large number of important memoirs have 

 been published, containing the results of embryological investigations 

 upon Cirripedia and Isopoda. Attention may be drawn to those by 

 Theodore T. Groom (Phil. Trans. Roy. Soc, London, vol. clxxxv., pp. 

 119-232); by Carl Chun (Bibliotheca Zoologica, heft xix., pp. 77-106; 

 Stuttgart, 1895) ; by Carl Aurivillius (Kongl. Svenska Vetenskaps 

 Akademiens Handlingar, vol. xxvi., no. 7, pp. 1-89); and by J. P. 

 McMurrich (Journal of Morphology, vol. xi., pp. 63-154). 



Of Mr. Groom's conclusions one of the most striking is this, that 

 probably throughout the Cirripedia thoracica the structure of the 



