126 BRITISH PALEOZOIC SPONGES. 



The only form with which the present species can be compared is that described 

 by the late Dr. Holl 1 under the name of Protospongia maculceformis. Judging from 

 the short description, unaccompanied by a figure, this form, which occurs under 

 similar conditions of preservation, belongs to the genus Phormosella. It is about 

 as large again as P. ovata. Unfortunately the type specimen has been lost, and no 

 other is known. 



Phormosella ovata is of rare occurrence, but judging from the number of 

 individuals associated together on the same slab as represented by fig. 2, it appears 

 to have been of a gregarious habit. The type specimen is in the Museum of the 

 Geological Survey, Jermyn Street. 



Distribution. — Silurian : Aymestry Rock, Mocktree, Shropshire. 



Genus. — Dictyophyton, Hall. 

 1863. Sixteenth Annual Keport State Cabinet, New York, p. 87. 



Syn. — Hydnoceras, Conrad; Tetragonis, M'Coy, Salter, F. Boemer (in part). 



Generic Characters. — Cylindrical, prismatic, or cup- or vase-shaped Sponges, 

 probably free, since neither stem nor anchoring appendages have been discovered. 



The walls consist of a connected spicular framework disposed so as to form 

 rectangular, quadrate, or oblong areas, which may be subequal or of larger and sub- 

 ordinate squares. The character of the spicules of the framework is not fully 

 known, they appear to be arranged in bundles, but whether these bundles are com- 

 posed of cruciform, or merely rod-like, spicules has not yet been determined. A 

 thin membrane appears to have extended over the area between the framework. 

 The mode of union of the spicules is not definitely known. 



The type of the genus was regarded by Conrad 2 as representing a sub-genus of 

 Orthoceras, which he named Hydnoceras tuberosum. Subsequently Prof. James 

 Hall believed it to be the frond of a marine alga, and with Conrad's consent 

 changed the generic name to Dictyophyton. In 1879 the late Prof. Schimper 3 

 expressed doubts as to its plant nature, and stated that its structure much nearer 

 resembled the skeleton of siliceous Sponges. Later, Ferd. Roemer 4 pointed out the 

 similarity of its structure to that of Tetragonis Danbyi, M'Coy. After this, Mr. 

 R. P. Whitfield 5 compared this genus, with other allied forms, to Sponges like the 



1 ' Geol. Mag.,' vol. ix (1872), p. 350. 



a ' Journ. Nat. Sci. Phil.,' vol. viii, p. 267. 



3 'Zittel and Schimper's ' Handb. der Pal.,' Bd. ii, Lief. 1, p. 69. 



* « Letha;a pal.,' Th. i, p. 127. 



5 ' Amer. Journ. of Science,' vol. xxii, p. 53. 



