ON THE BEITISH SPECIES OF CRISIA. 153 



fically distinct, and a more careful examination of C. cornuta 

 has convinced me that S mitt's statement may be explained 

 in such a way that it is unnecessary to follow him in his con- 

 clusion. 



Even if the two forms are not really distinct, it appears to 

 me worth while to call attention to what would then be an in- 

 teresting case of a definite variation of the ovicells correlated 

 with the presence or absence of spines on the zooecia. 



C. geniculata consists typically of a series of iuternodes, 

 each of which is composed of a single zocecium ; from opposite 

 sides of this zooecium arise a pair of branches which are not quite 

 at the same level. An excellent figure of this form is given in 

 Busk's ' British Museum Catalogue ' (part 3), pi. i, fig. 2. On 

 comparing this figure with fig. 7 on the same plate (representing 

 C. cornuta), it will be seen that C. cornutaexactly resembles 

 C. geniculata as far as those zocecia which bear two branches 

 are concerned; but that, as Busk points out (p. 3), one of the 

 branches is usually replaced by a jointed spine. 



It is obvious that spines will be absent in C. cornuta if 

 two branches are developed from each zooecium. Further, the 

 spines are very readily broken oflF, and a close examination is 

 then sometimes necessary to discover the small basis with which 

 the spine articulates. From one or other of these causes I have 

 severaltimes observed branchesof normal coloniesof C. cornuta 

 having a close resemblance to C. geniculata; and this may 

 be the explanation of Smitt's statement referred to above. 



In every case in which I have observed the ovicells — although 

 I must add that I have not obtained many ovicells of C. geni- 

 culata — I have noticed the following characteristic differences 

 between the two forms ; a reference to Busk's figs. 2 and 10 

 (1. c, pi. i) shows that the forms examined by Busk were 

 similar to those which I have myself found. Busk's fig. 4 (C. 

 geniculata) does not, however, quite agree with the specimens 

 which I have examined. 



The ovicell of C. cornuta (fig. 9) is the basal and only 

 member of its own internode ; it bears a lateral branch on each 

 side, these branches originating at not exactly the same level. 



