RECENT RESEARCHES IN THE JUAtOM ACE Ai. 81 



to be its true significance to this admirably worked-out form, 

 Rhahdopleura, and that I conceive to depend upon the large 

 development of that most Molluscan of organs, the " foot." 

 The absence of a mantle-fold is of small significance, being 

 paralleled in various Mollusca proper. The abortion of the 

 cephalic region is characteristic of the whole branch of 

 Acephala, and the same region suffers in Dentalium, and 

 even in the Cephalopoda also, to the advantage of the ever- 

 more function-annexing foot. 



Recent Researches in the Diatomace-s:. 

 By Rev. E. O'Meara, A.M.i 



VI. 



In the family Achnanthece the valves are symmetrical in the 

 longitudinal axis, but unsymmetrical in the plane of separa- 

 tion ; the frustules are more or less geniculate, so that of the 

 valves one is concave while the other is convex ; the former 

 only possesses a central nodule. The genera Achnanthidium 

 and Cocconeis agree in these general characteristics, but are 

 separated into a distinct family, the Cocconeidece, for reasons 

 that shall be hereafter assigned, so that the family Achnanthece 

 embraces the solitary genus Achnanthes. The two species 

 of this genus Avhicli have been observed abundantly in a living 

 state, A. bi'evijjes and A. subsessilis, correspond with the 

 Navicidece in the structure of the cell-contents, inasmuch as 

 they possess a middle granular plasm-mass and two endo- 

 chrome-plates lying on the girdle-bands, and thence passing 

 over the valves. The endochrome-plates exhibit a slit in the 

 middle, and separate by an incision proceeding from the ends. 

 The well-defined nucleus lies always nearer to the concave 

 than to convex valve. In the few specimens of A. longipes 

 which came under Dr. Pfitzer's notice, the endochrome-plates 

 Avere split up into numerous small pieces ; but whether this 

 be the normal condition or not remains to be determined. 



The marine A. longipes was observed by Smith in the act 

 of forming auxospores, the same form, as well as A. subsessilis, 

 by Liiders. In respect to the former Smith maintained that a 

 single cell forms two auxospores, Liiders supposed that two 

 cells co-operate to produce the same result ; while in the case 

 of A. subsessilis a single mother-cell gives birth to a single 

 auxospore. In both cases, according to Liiders, the cell- 

 1 Continued from Vol. XIII, p. 15. 



VOL. XIV. NEW 8EH. 1' 



