122 



HARDWICKE'S SCIENCE-GOSSIP. 



an annular membrane, and when in situ, the valves 

 are placed so that the membranes are in contact ; 

 and thus the space included between the two dis- 

 coid valves is partially divided into two (not three) 

 chambers. The membrane is so thin that when 

 mounted in balsam it is not visible. I have, how- 

 ever, been able to detect it upon one or two occa- 

 sions." This annular valve is of course the cingulum 

 or connecting-zone, and no membrane is present. 



The description of the valves is more accurate. 

 " The two parts of the disks consist of, firstly, a very 

 thin membrane, somewhat flexible and elastic, and 

 capable of resisting the action of boiling nitric 

 acid, and on this membrane are the characteristic 

 spider's-web-like markings which have given rise to 

 the name of the genus ; secondly each disk is com- 

 posed of a siliceous framework admirably adapted 

 to support and strengthen the outer membrane, and 

 bears a very close resemblance to a circular Gothic 

 window." 



The author is mistaken in supposing that the 

 thin valve is uppermost ; the upper valve is stout, 

 and the so-called siliceous framework, so far as I 

 can make out, is an integral portion of it. The 

 thin valve is below, and is probably an early state 

 of a newly-formed valve, aud which will, after self- 

 division has taken place, form the opposite valve of 

 the frustule. 



The species of Arachnoidiscus hitherto described 

 are the following :— A. Ehrenbergii, A. ornatus, A. 

 Grevilleamis, A. Hardmanianus, A. Indicus=A. Eh- 

 renbergii. The form figured and described by Mr. 

 Shadbolt is the second in the list, and is apparently 

 more widely distributed than A. Ehrenbergii. It is 

 distinguished from the latter by the transverse 

 costse between the radiating lines ; those near the 

 margin are irregularly branched, resembling the 

 venation of the leaves of a dicotyledonous plant ; 

 the spaces between the costse are delicately punc- 

 tate, the puncta becoming larger as they approach 

 the centre. = Hemiptychus ornatus (Ehrenberg), 

 Arachnoidiscus Japonicus (Shadbolt), A. Nicobaricus 

 (Ehrenberg), A. ornatus (C. Janisch, in"Zur Charak- 

 teristik des Guano's von versckiedenenFundorten," 

 p. 12, Taf. 1, fig. 3. South Africa, Nicobar Islands, 

 West Coast of S. America. 



Arachnoidiscus Ehrenbergii may be easily recog- 

 nized by the absence of the transverse costse, the 

 large irregularly - shaped granules between the 

 radiating lines. These granules, when examined 

 with oblique light, appear to consist of closely- 

 packed beads. A.Indicus, Ehrenberg, "Microgeo- 

 logie," Puget's Sound, Monterey Stone, Vancouver's 

 Island. 



The form figured in the Synopsis is probably the 

 above, but there can be little doubt that it never 

 lived in any British locality; moreover, the form 

 figured was not from the photograph of De Bre- 

 bisson's. Tuffen West told me that the drawing 



was made from an actual specimen. This fact 

 accounts for the non-agreement of Smith's generic 

 characters with the figure.* 



Bailey in a letter to Dr. Arnott (Walker Amott 

 on Arachnoidiscus, Quart. Journ. Mic. Soc, vol. iv. 

 p. 161), says, "I see that Smith in his Brit. Diat. 

 gives me as the founder of the genus. This is not 

 correct ; but the species is mine, and is very different 

 from A. Japonicus, with which Smith confounds it." 



Arachnoidiscus Grevilleanus is a rare species 

 occurring in the Barbadoes deposit : it possesses 

 the radiating cosfse characteristic of the genus, 

 which reach nearly to the centre of the disk ; alter- 

 nating with these are shorter rays, above one-third 

 of the length of the principal rays, but the central 

 smooth space surrounded by a circle of elongated 

 cellules is wanting, the centre being occupied by a 

 little star composed of five or six minute elongated 

 cellules. The margin of the valve has several rows 

 of small cellules, which become larger midway 

 between the centre and margin, aud again becoming 

 smaller aud scattered as they approach the centre. 

 (Greville, in Trans. Royal Mic. Soc, vol. xiii. p. 47, 

 pi. v. fig. 7.) 



The last species of this genus was placed by 

 Dr. Greville in his genus Stictodiscus, but its 

 affinities, judging from his figure and description, are 

 with the present genus. As I have never seen this 

 species, I can only give Greville's specific characters. 



" Stictodiscus Hardmanianus. — Large ; radiating 

 compartments numerous, reaching nearly to the 

 centre, with five to six rows of minute puncta at the 

 base (margin of valve), followed by a single row of 

 pseudo-pores; centre occupied by two circles of 

 granules, and a minute cluster at the umbilicus; 

 diameter -0050". Monterey deposit. Mr. Hardman." 



It will be seen from the above description that 

 this species only differs from the type forms in the 

 absence of the circle of elongated cellules, and 

 which are represented by a circle of large monili- 

 form granules. 



The probability of the specimen of Arachnoidiscus 

 being a British form is to my mind very doubtful, 

 although two cases have been published of its 

 occurrence ; viz., one frustule by De Brebisson, as 

 cited in the Synopsis, and one by Captain Hutton, 

 F. G. S. {Quart. Journ. Mic. Science, vol. v. n.s., 

 p. 132). If this form had really lived on our coasts, 

 more than three specimens would have been dis- 

 covered in thirty years. Captain Hutton's specimens 

 were supposed to have been in a gathering made 

 from a small brackish waterpool at Malahide, but 

 were probably attached to the test-tube in which 

 the gathering had been boiled. The writer remarks 

 that he had not cleaned any material containing 



* Frustules adherent, disciform ; valves plane or slightly- 

 convex, cellular, marked with concentric and radiating lines ; 

 pseudo-nodule central, conspicuous. 



