ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 641 



in regard to the degree of concentration requisite, the development 

 might be carried further. 



Systematic Position of the Solenogastres.*— J. Thiele contributes 

 a detailed discussion of this problem, and comes to the conclusion that 

 the Solenogastres represent a group of worms most nearly related to 

 Gordiidae and Annelids. The relation of the uterus to the heart and 

 the beginnings of a radula-formation bring them in contact with the 

 Molluscan stock, among which the Chitonidas with their persistent 

 lateral cords are nearest to them. The memoir includes a lengthy 

 anatomical description of the Solenogastres, — Notomenia clavigera, 

 Proneomenia australis, P. sluiteri, Neomenia carinata, and Chsetoderma 

 nitidulum. 



Variation in Position of Adductors in Anodonta grandis Say.f — 

 E. E. Downing has measured a large number of specimens and reached 

 the following results. (1) The distance of the adductor muscle from 

 the umbo, measured along the line of migration, varies considerably in 

 its relation to the distance to the shell margin, measured along the 

 same line produced. (2) While the range of variation is comparatively 

 large, the frequencies are well concentrated at or near the mode. In 

 93 p.c. of the cases the position of the muscle would not vary 3 mm. 

 in either direction from the mean position. (3) Roughly speaking, we 

 may say that the muscle is situated two-thirds of the way from the 

 umbo to the margin. Accurately, the mean ratios are 0*6757 for the 

 anterior muscle, and 0*6608 for the posterior. (4) The anterior adductor 

 tends to move toward the margin at a more rapid rate than the posterior, 

 although there is a tendency in both to lag behind the rate of growth 

 of the shell margin. We seem to have in this form a species that is 

 stable, rather than one that is in process of rapid evolution. 



Structure of Gills of Lamellibranchs.J — W. G. Hidewood has 

 examined 215 species, belong to 118 genera, and finds that, except in 

 a very broad way, the minute structure of the gill, like the grosser 

 structure, cannot be taken as an indication of genetic affinity. 



Three main types of gill structure can, however, be recognised, re- 

 presenting apparently three successive grades of complexity. Nuculidae 

 and Solenomyidse (Pelseneer's Protobranchia) are marked by the mutual 

 freedom of the gill-leaflets or platelets into which the embryonic gill 

 papillea expand. In the two other types the embryonic papillae elongate 

 into filaments, which are held in juxtaposition by interlocking cilia 

 disposed in circular patches on the anterior and posterior sides of the 

 filaments (tleutherorliabdic), or by regularly arranged horizontal bars of 

 cellular tissue, which put the adjacent filaments into organic connection 

 the one with the other and convert the interfilamentar spaces into rows 

 of fenestras (synaptorhabdic). 



Pelseneer's order Pseudolamellibranchia is regarded as ill-founded. 

 Evidence is also adduced to show that Pelseneer's Septibrauchia are 

 degenerate forms of the Lyonsiella type, and the suppression of Septi- 

 branchia as a distinct order is advocated. 



* Zeitschr. wiss. Zool., Ixxii. (1902) pp. 249-466 (10 pis. and 21 figs.), 

 t Amer. Nat., xxxvi. (1902) pp. 395-400 (6 figs.). 

 X Proc. Roy. Soc. London, lxx. (1902) pp. 499-5(0. 



December 17 1\ 1902 2 x 



