178 



In agreement with Stein (1883), Biitschli (1883—1887) and 

 Schutt (1895) as opposed to Klebs (1884) I regard the plates of the 

 Dinoflagellates as structurally stable features, subject, of course, to 

 variation and abnormalities, and even to irregularity in a few species, 

 but of sufficient constancy to afford a most satisfactory basis for classi- 

 fication, a basis, moreover, now widely adopted as a criterion for generic 

 distinction among the Dinoflagellata. 



As Schutt (1895) has so well shown, the superficial lists, even the 

 primary ones, are in no way a safe criterion for the analysis of the 

 thecal structure. This can be accurately determined only by following 

 the sutures. These, however, are obscure, and are often only dimly 

 suggested by vaguely outlined overlapping margins of the plates that 

 appear best upon deep focus on the thecal wall. Prolonged treatment 

 in warm alkalis does not always bring about separation of the plates 

 along the suture lines though it is of great assistance in resolving the 

 structure. 



Actual separation of the plates in situ is the only safe guide to an 

 analysis of the thecal wall, but a failure to cause separation under treat- 

 ment does not afford a valid basis to deny the existence of a suture, 

 since it may be merely the result of the condition in which the material 

 is found at the time of examination. I am inclined to believe, as a result 

 of many tests, that all reports of three apical plates in Ceratiimi rest upon 

 material imperfectly separated, and that the variation in the splitting 

 of the apical and left antapical horn reported by Klebs (1884) is more 

 a matter of imperfect separation than of a morphological variation in 

 suture lines and number of plates. In other words I have found Ceratium 

 far more constant in the matter of sutures and plates than it has been 

 reported to be. 



Stein (1883) characterized the genus and distinguished it from Peri- 

 diniuni^ with which Ehrenberg (1835) had confused it, and from which 

 Claparèdeand Lachmann (1858 — 1859) had subsequently separated 

 it on the basis of the horns, by its plates, which he defined as composed 

 »aus 3 Basalien und 3 Frontalien am Vorderleibe, und aus 3 Basalien 

 und einer Endplatte am Hinterleibe«. Bütschli (1883 — 1887), probably 

 influenced by Klebs' (1884) discovery, regards the theca as composed 

 anteriorly of three equatorial and three (sometimes more) apical plates, 

 and posteriorly by three equatorial and one apical plate. Schutt (1896) 

 regards the »Oberschale« as composed of three intermediate plates and 

 four end plates and the »Unterschale« as made up of three equatorial 

 plates and one end plate. He notes also the presence of several girdle 

 plates. 



Entz (1905) was the first investigator to establish the existence of 



