254 Dr. Greville on some new species of Sargassum. 
plates pierced each with six foramina; mouth and anus small, 
both central. | 
This genus is remarkable for the irregularity of form and size — 
of the interambulacral plates, differing in this both from Archeo- 
cidaris and Palechinus ; from the former it also differs in the 
greater number of the interambulacral plates being destitute of 
the mammillated primary tubercle, and by its small size and 
lateral position on those plates which do bear it ; from Palechinus 
it differs, besides the above, in the two rows of primary tubercles 
to each interambulacrum, &c. I at present know but one species. 
Perischodomus biserialis (M‘Coy). 
Sp. Char. Diameter (of flattened specimens) about 24 inches, 
width of ambulacra at middle 3 lines; width of mouth and 
ovarian circle each about 3 lines; granules on the five rows of 
irregular interambulacral plates scarcely visible, the two rows 
of mammillated and perforated primary tubercles bordering 
the ambulacra very small ; two rows of ambulacral plates, about 
six or seven occupying the same space as one of the interam- — 
bulacral plates of the middle of the row. 
Some few of the ambulacral plates are wedge-shaped, pointed 
towards the interambulacra, as in the sketch. The primary 
spines, as far as seen, were cylindrical and smooth. 
Rare in the lower carboniferous limestone of Hook Head, Wex- 
ford. 
(Col. University of Cambridge (anal and genital half), and 
Dr. Griffith at Dublin (oral half).) 
XXIX.—Alge Orientales :—Descriptions of new Species belonging 
to the genus Sargassum. By R. K. Grevitiz, LL.D. &ec.* 
[Continued from p. 219.] 
[| With a Plate. } 
16. Sargassum squarrosum (nob.); caule filiformi, angulato; foliis 
(parvis) anguste obovatis, obtusis, plus minusve repando-dentatis ; 
vesiculis subsphericis, brevissime petiolatis ; receptaculis obovatis 
vel lineari-oblongis, plano-compressis, acute lateque dentatis. 
Hab. in mari Peninsule Indiz Orientalis ; Wight. 
Root I have not seen. Stem filiform, angular, a foot to, pro- 
bably, a foot and a half long, bushy with numerous branches 
which appear to be generally 2 or 3 inches long. Leaves 
small, half an inch or, rarely, three-fourths of an inch in length, 
* Read before the Botanical Society of Edinburgh, February 8, 1849. 
