GENUS CERATIUM, 455 



represented. Of C. tripos the variety macroceros is most plentiful, the majority of 

 examples being remarkable for the extreme length and slenderness of their arm-like 

 processes, which in this respect not unfrequently exceed the proportions given at 

 PI. XXV. Fig. 24. Every gradation from this attenuate variety to the more typical 

 condition, Fig. 33, was encountered, but in no instance the thick-armed variety 

 arcticum, whose appendages are apparently specially adapted to preserve the equili- 

 brium of the animalcule in the colder and consequently denser waters of the Arctic 

 seas. An interesting structural feature was elicited in connection with the author's 

 examination of this type. In specimens mounted in the usual manner for future 

 observation the median posterior process was shown to be composed of four 

 longitudinal segments ; these under pressure became separated from one another 

 throughout the greater portion or even their entire length, presenting under such 

 conditions the aspect of a fascicle of four rigid, distally divergent setae. This 

 opportunity of becoming personally acquainted with the typical pelagic repre- 

 sentative of the genus Ceratium was also utilized by the author for the solution 

 of the moot question concerning the composition of its exceedingly brittle indu- 

 rated carapace or cuirass. It had been previously suggested by Mr. Charles 

 Stewart, of St. Thomas's Hospital, with reference to the accredited capacity of 

 this element to withstand incineration, that it was probably of a siliceous nature, 

 such opinion being endorsed by the author in an account given in the ' Midland 

 Naturalist ' for May 1880, of various marine Infusoria collected off the coast of Corn- 

 wall. The views there expressed, based only on unsupported testimony, are now 

 entirely discarded, for, on putting the matter to the test by heating examples on 

 talc over a spirit-lamp, it was found that they speedily shrivelled up, and ultimately 

 disappeared, while the more delicate siliceous diatom-frustules and sponge-spicules 

 scattered beside them remained intact. The possibility of the carapaces of these 

 Ceratia being composed of silex was thus disproved, and no grounds are left for 

 supposing that their composition is other than corneous or chitinous, as obtains in the 

 carapaces, loricag, or capsular elements of all the more familiar infusorial species. 

 Yet additional testimony in support of the view now advocated is afforded by the 

 fact that the remains of several reputed species of Ceratium, C. pyrophorum and 

 C. delitiense, have been obtained as fossils in flints belonging to the cretaceous 

 formation. It is a well-known fact to geologists that silica in its first state of com- 

 bination is entirely eliminated from these deposits, and is present only in its recom- 

 bined and concrete form. It may be hence consistently maintained that the Ceratia 

 met with in the chalk could not have possessed siliceous tests. 



Ceratium furca, Ehr. sp. PL. XXV. FIG. 31. 



Cuirass subtriangular, longitudinally striate, bearing three horn-like 

 processes, the two antero-lateral ones directed straight forwards, not 

 exceeding the body in length ; the median posterior process straight and 

 slender, twice the length of the anterior ones. Entire length 1-120". 



HAB. Salt water. Colour yellow. 



Claparede and Lachmann include in this species both the Peridinium furca and 

 P. lincatum of Ehrenberg. The dimensions of the last-named type are however 

 so much smaller, 1-380" only, that it seems highly probable that the two are distinct. 

 The present form, according to the last-named authority, is eminently phosphor- 

 escent. Although usually regarded as entirely marine, M. Werneck has reported 

 the occurrence of an apparently identical species in fresh water in the vicinity of 

 Salzburg. 



Ceratium biceps, C. & L. 



Cuirass subtriangular, striate longitudinally, bearing three horn-like 

 processes, neither of which exceed the length of the body, the two anterior 



