76 Roper, on the Diatomacece of the Thames. 
apices being very short ; the length is 1-5 8 8th to 1-7 7 9th of 
an inch, and breadth l-1034th to l-968th of an inch. I 
should refer it to Rhaphoneis rhombus. The only point of 
distinction between this genus and the Doryphora of Pro- 
fessor Kiitzing appears to be the presence of a stipes ; and it 
would be a point of some interest to determine whether 
these forms are attached in a similar manner, or whether, as 
I imagine from the abundance with which they occur, and the 
absence of any direct negative observations, the frustules are 
free as in Navicula. 
A large and well-marked species is represented by fig. 5, 
which has not, I believe, been hitherto figured as British. I 
have been unable to obtain a front view of a perfect frustule, 
though the single valves are by no means uncommon. By a 
comparison with some specimens of Zijgoceros rhombus from 
Petersburg, Virginia, kindly lent me by Professor Quekett, 
I have little doubt that it can safely be referred to that 
species,* as the only difference is, that in the Thames spe- 
cimens, the side view of the valves is rather broader in 
proportion to the length. The valves are nearly rhomboidal, 
slightly produced at the extremities, and terminate in a pro- 
jecting tubular horn or spine. The surface is minutely 
punctate with small hexagonal cells, radiating from the 
centre, and has from three to six small spinous processes at 
the sides, with two rather longer at the extremities of the 
valve. The length varies from l-300th to l-183rd of an inch, 
and breadth from l-375th to l-260th of an inch. 
Figs. 11 and 12 are, I believe, front and side views of 
Zygoceros surirella of Ehrenberg. I have only met with one 
specimen of the perfect frustules, represented by fig. 14, which 
agrees in form with the figure given by him in the ' Berlin 
Transactions' for 1839, tab. 4, fig. 12, and shows the smooth 
central band and striations, which distinguish the side view. 
Fig. 16, which I consider the side view of a larger specimen, 
somewhat resembles the genus Rhaphoneis, but differs, in 
the markings being nearly parallel, and though granular, so 
confluent as almost to appear as lines ; the central smooth 
portion terminates in two lobes, corresponding with the pro- 
jections, which appear at the extremities when the front view 
is obtained. The length is l-714th to l-1240th of an inch, 
and breadth l-1500th to l-2500th of an inch. I have met 
with the same species in the deposits of Pembroke Harbour. 
Fig. 6 a and b represents a small cross-shaped valve that 
occurs sparingly, which Mr. Smith, from a drawing, thought 
* The genus Zygoceros is included by Mr. Smith in that of Biddulphia ; 
this will, therefore, be the BidduljjJiia rhombus of the ' Synopsis.' 
