GENUS AMPHIDINIUMPROROCENTRUM. 461 



equalling in breadth one-half of the diameter of the body, traversed by two 

 or three spine-like linear thickenings ; the entire surface of the posterior 

 segment distinctly shagreened ; the anterior segment concave, operculum- 

 like, dilated, its margins delicately striate in a longitudinal direction. 

 Length 1-212". HAB. Saltwater: Adriatic. 



This species has been obtained sparingly among the material originally collected 

 at Fano, in the Adriatic, by Count Castracane, and recently placed at the author's 

 disposal by Dr. Wm. J. Gray, F.R.M.S. From all the species previously described 

 it may be distinguished by the distinct narrower prolongation of the anterior 

 segment, which is, as it were, added on to the ordinary ventricose body of D. 

 norvegica or D. acuminata, and contributes substantially to its total length. The 

 ventral plates are also of much larger comparative size, resembling, in connection 

 with the two or three perpendicular spine-like linear thickenings, the elevated 

 dorsal fin of some Acanthophrygian fish. The smooth narrow ridge developed 

 along the dorsal margin of the body is undoubtedly homologous with the elevated 

 crest distinctive of this region in D. norvegica. 



GENUS VIII. AMPHIDINIUM, C. & L. 



Animalcules encuirassed, having a transverse annular, ciliated furrow 

 situated at the posterior extremity, as in the genus Dinophysis, the pos- 

 terior segment being in a similar manner almost entirely atrophied ; the 

 longitudinal ventral furrow, out of which the flagellum springs, represented 

 only towards the anterior extremity, and not joining the transverse annular 

 one ; no salient ventral plates. Inhabiting salt water. 



Amphidinium operculatum, C. & L. PL. XXV. FIGS. 44-46. 



Body oval, depressed, the ventral side slightly flattened, the dorsal one 

 convex ; posterior segment rudimentary, operculum-like ; the larger anterior 

 portion frequently containing a central dark brown structure, possibly the 

 endoplast, with radiating prolongations ; surface of cuirass entirely smooth. 

 Length 1-503". HAB. Salt water : Norwegian coast (C. & L.). 



Claparede and Lachmann found in company with the normal animalcules, as 

 above described, smaller perfectly transparent examples, having in some instances a 

 pyriform, and in others an almost orbicular contour; these they regard as mere 

 younger conditions of this species. 



GENUS IX. PROROCENTRUM, Ehrenberg. 



Animalcules encuirassed, cuirass consisting of a single continuous piece, 

 not divided as in the preceding genera by a transverse annular furrow ; 

 a stiff spine or tooth-like process usually developed at the anterior extremity 

 close to the insertion of the single flagellum. Inhabiting salt water. 



Prorocentrum micans, Ehr. PL. XXV. FIGS. 37-39. 



Cuirass oval, compressed, pointed posteriorly, the ventral border 

 flattened, the dorsal one convex, the anterior margin truncate, armed near 



