CARADOCIAN CYSTIDEA FROM GIRVAN. 427" 



and Irregularia, of which the former corresponds with my Family Glyptocystidae, and 

 the latter comprises the Caryocrinidae, Echinosphaeridae, and " ? Fam. Tetracystidae." 

 This last is the equivalent of my Tiaracrinidae, and must retain that name, not merely 

 on grounds of priority, but because there is only one genus Tetracys/is, and that was 

 established in 1904 by C. Schuchert for one of the Apiocystinae {seu Lepadocystinae). 



§ 265. The names Regularia and Irregularia, indicating the disturbance of peiita- 

 merism by an anal plate in the latter, seem to me peculiarly inappropriate ; for the 

 ordinary observer will scarcely suppose that so irregular a form as Pleurocystis is 

 one of the Regularia, or that so regular a form as Caryocriiius is one of the Irregularia. 

 Neither am I inclined to accept Prof Jaekel's view that his Irregularia are specialised 

 descendants of his Regularia. 



There is much to be said for a taxonomic association of the genera included in 

 Jaekel's Regularia, and I attained this end by keeping them in a single Family 

 divided into Subfamilies. Prof Jaekel prefers to make of them a Suborder divided 

 into Families. The difference is one of degree, not of principle. 



§ 266. But as for the Irregularia, Prof. Jaekel himself admits (1900, p. 291) that 

 they form " keine festgefligte Einheit wie die Regularia," and he does not even attempt 

 a diagnosis. His reasons for instituting a Suborder appear to be two : first, that if 

 the other genera be placed in one Suborder, then for purely practical convenience these 

 must be placed in another ; secondly, that it is possible, or, at any rate, has not been 

 proved impossible, that the three Families of the Irregularia, however independent in 

 their origin, still did arise, by sudden changes of similar character, from an ancestor 

 within the Regularia. Personally, I do not admit the probability of this ; but even 

 if I did, I should not consider it a very good basis for a Suborder. 



It seems to me that either we must refrain for the present from splitting the 

 Rhombifera into Suborders ; or, if we are impelled to follow Prof. Jaekel in turning 

 the Glyptocystidae into a Suborder, then we must take the same action with the 

 Caryocrinidae, the Echinosphaeridae, and various other Families or groups of Families. 

 But our knowledge is certainly not enough to enable us to pursue the latter course with 

 any surety. Therefore the only line of safe progress is to consider whether it might 

 be advisable to split up any of the existing Families. 



§267. That the Glyptocystidae (= Regularia) form a closely related group has 

 been independently maintained by Prof. Jaekel and myself. Further than this, we 

 were both led to reconstruct an identical ancestor or archetype for the whole group, 

 and to fix on Mimocystis as the actual fossil most closely resembling this (see Jaekel, 

 1899, f 37 ; Bather, 1900, p. 59, f. xxi.). 



§ 268. Here, once for all, let me explain clearly that though Prof Jaekel and I 

 have constantly been for the past twenty -five years in the most friendly communica- 

 tion, still, when we were at the same time time preparing our two big books on this 

 subject, we could not mutually discuss every new opinion. Each of us had his text 

 already in type before the publication of the other's work. Thus, while Dr Jaekel 



