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 interarabulacral plates, differing in this both from Archao- 

 cidaris and Palachinus ; 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 Palcechinus 

 it differs, besides the above, in the two rows of primary tubercles 

 to each inter ambulacrum, Sec. I at present know but one species. 



Perischodomus biserialis (M'Coy). 



Sp. Char. Diameter (of flattened specimens) about 2^ 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. — Algce Orientales : — Descriptions of new Species belonging 

 to the genus Sargassum. By R. K. Greville, LL.D. &c.* 



[Continued from p. 219.] 



[With a Plate.] 



16. Sargassum squarrosum (nob.); caule filiformi, angulato ; foliis 

 (parvis) anguste obovatis, obtusis, plus minusve repando-dentatis ; 

 vesiculis subsphaericis, brevissime petiolatis ; receptaculis obovatis 

 vel lineari-oblongis, plano-compressis, acute lateque dentatis. 



Hab. in marl Peninsulse Indiae 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 tlie Botanical Society of Edinburgh, February 8, 1849. 



