920 



STSTEIIATIC HISTORY OF THE INFUSORIA. 



tiise ends, and a slight, transvei*se cen- 

 tral depression } median line stout, nearly 

 central ; striae oblique, 40 in -001". = 

 P. lanceolatmn, Norman, MS. Marine. 

 Europe. Foimd in nearly every gather- 

 ing from deep water, and in stomachs 

 of Ascidians, Noctilucse, Pectens, &c. 

 Colour ta^\^ly brown ; -0048" to -0110' ; 

 median line scarcely flexed, except near 

 the ends. The description is by George 



Norman, Esq. 



P. aciitum (Norman, MS.^ — Large, 

 broadly lanceolate, elongated, moderately 

 flexed, gradually tapering to the very 

 acute apices ; median line delicate, much 

 flexed, not central ; strise oblique, 50 in 

 •001" Marine. Stomachs of Ascidians, 

 &c. Europe. Very pale straw-colour or 

 nearly hyaline ; -Oil" ; median line 

 flexed throuo-hout. 



Genus TOXONIDEA (Donkin). — Erustules simple, free ; lateral valves 

 elongated convex, with the sides not symmetrical ; median line arcuate, with 

 central and terminal nodules, its ends curved towards the same margin ; 

 stride obhque. Marine. Toxonidea is closely allied to Pleiu-osigma ; indeed 

 the forms placed here are regarded by Professor Amott as distorted species 

 of that genus. The absence, however, of a sigmoid flexure, both in the valves 

 and median line, is so different from what we find in Pleurosigma, that we 

 think it advisable to admit Toxonidea until Dr. Donkin's views are disproved 

 by more perfect observation. 



Toxonidea Gregoriana ("Donkin). — 

 Large, lanceolate, with the obtuse ends 

 curved upwards ; median line concurrent 

 with the lower margin near the ends ; 

 strise tine, oblique, about 50 in -001". 

 TM. vi. p. 19, pi. 3. f. 1. Britain. Straw- 

 coloured ; -008" to -009" ; median line 

 curved upwards near the end, and " re- 

 sembling the figm-e of an unbent Scy- 

 thian bow ; " dorsum rather more con- 

 vex than the venter. 



T. insignis (Donkin). — Arcuate or 

 semi-lunate, with produced, subacute 

 ends ; median line not central, strongly 

 arcuate ; striae very fine, about 75 or 80 

 in -001". TM. Yi. p. 21, pi. 3. f. 2. -0048" 



to -006" ; " valves resembling a straw- 

 coloured strung bow or a cocked hat," 

 with very convex or gibbous dorsum and 

 straight venter. Professor Arnott re- 

 gards this species as a distorted state of 

 Pleurosigma .^Jstuaru. Britain. 



T.midulata (Norman, 3IS.). — Arcuate, 

 with three slight dorsal undulations, ob- 

 tuse somewhat recurved apices, and con- 

 cave venter gibbous at its centre ; striae 

 oblique, 50 in -001". From Ascidians. 

 North Sea. Very pale straw-colour, Tvath 

 pinkish reflections ; '0055" j longitudinal 

 suture concm-rent with the ventral mar- 

 gin except at the inflated centre, (viu. 

 46.) 



Genus DONKINIA (n. g.). — Erustules simple, free ; front view panduri- 

 form, as broad as the lateral view ; valves convex, keeled, with nodules and 

 decussating striae as in Pleurosigma. Marine. We have constituted this 

 genus for the reception of some Diatoms possessing characters intermediate 

 between Pleurosigma and Amphiprora, and have much pleasure in dedicating 

 it to Dr. Donkin, who, amongst his many interesting discoveries, first directed 

 attention to several of the species placed in it. Dr. Donkin refen^ed the 

 species to Pleurosigma on account of the similarity of striation ; but they 

 dlfi'er from that genus in the broad, constricted front view ; and from these 

 characters, together with their very convex, keeled valves, we were induced 

 to regard them as more nearly allied to Amphiprora. Indeed there is 

 little essential difference between keel, crest, and wing, these being, in our 

 opinion, merely different stages of development. This opinion was also 

 adopted by Professor Amott, whose critical kno^^dedge of genera commands 

 the highest deference. The species placed in Donkinia differ from Amphi- 

 prora, not only in their decussating striae (a character sometimes difficult to 

 verify, and of rather doubtful generic value), but also, according to Dr. 

 Donkin, in the absence of lateral wings to the valves. 



Donkinia cristata (E.). — Narrow-lan- 

 ceolate, gradually subulate at each end ; 



central nodule transversely oblong ; me- 

 dian line sigmoid, crested. = Navicula 



