Microscopical Society. 2673 



for a short time it again made a spring forwards, and so on at short intervals until it 

 had entirely crossed the field. This was repeated several times, with the same result. 

 Suspecting, however, that this mode of progression hy starts could not be natural, he 

 made some alteration in the arrangement, and then found that the motion forwards 

 became gradual and equable. The field of the microscope included rather more than 

 fjjth of an inch, and the apex of the grass traversed the whole diameter in somewhat 

 less than ten minutes. In reference to the mode of growth of this part of the plant, 

 he stated his opinion that a gradual expansion and elongation of the cells takes place, 

 causing the increase both in the length and breadth of the blade, but that there might 

 also be additional cells produced near the root, which cells may be gradually deve- 

 loped and matured in the stem during its growth : this he was inclined to think was 

 actually the case, because the outer edge of the grass was serrated at regular dis- 

 tances. When the upper surface or cuticle was brought into focus, certain more or 

 less hexagonal cells, coated with a very thin layer of silica, became visible : this outer 

 covering, however, does not appear to undergo the slightest change during the pro- 

 gress of the growth : the serrations also appeared to cover the whole surface of the 

 grass. Hitherto he had been unable to observe the development and growth of 

 cells near the root, his attention having been more particularly directed to the apex 

 of the blade, the pushing forward or growth of which he considered to be occasioned 

 not by the before-mentioned expansion or elongation of the terminal cells, but by the 

 addition of new matter to the base of the blade. 



Another paper, " On the Structure of the Siliceous Loricae of the Genus Arach- 

 noidiscus," by George Shadbolt, jun., Esq., was read. The author commenced by 

 giving a general account of the Arachnoidiscus, — which name was given to the object 

 under consideration by Mr. Deane, in consequence of the close resemblance of its 

 markings to a spider's web, — and then proceeded to detail his own observations on 

 the subject. These shells, although closely resembling bivalves, are not, in his 

 opinion, strictly so, but are more properly speaking multivalves, each shell consisting 

 of two circular discoid portions and two annular valves, exactly similar respectively to 

 each other. Each of the discoid valves is capable of further separation into two cir- 

 cular, but dissimilar, portions : this separation is exceedingly difficult to accomplish. 

 Mr. Shadbolt had, however, succeeded so far as to satisfy himself that the discoid 

 portions are thus composed, which may also be verified in another manner which he 

 subsequently described. The disks are composed, first, of a very thin membrane, 

 having on it the web-like markings before alluded to, which membrane is situated 

 internally ; secondly, of a siliceous framework, well adapted to support and strengthen 

 the outer membrane. These portions he was able to obtain separately, by boiling 

 them in nitric acid ; but it was in a few specimens only that the separation was ef- 

 fected, and it very rarely occurred that the siliceous framework was obtained entire. 

 He then described the mode of manipulation under the microscope, before alluded to, 

 in which, being viewed as opaque objects, the structure of the shells may be demon- 

 strated to be as he stated. When examined in this manner, there also appears to be 

 a central opening in each disk, which, when in situ, is partially covered internally by 

 a delicate cup-like process, so as to form a species of valve : this frequently prevents 

 the opening from being satisfactorily shown, and hence some doubt has arisen as to 

 its existence. The author then proceeded to offer various suggestions as to the uses 

 of the several parts. The paper was illustrated by accurate drawings of the parts 

 described, made by Mr. Legg. 



