270 Proceedings of the Royal Irish Academy. 



Scirpearia the largest are those of JS.anomala, viz., 0-15 mm. x0"034mm. ; 

 while in S. iKdiimta the largest are only 0'061 mm. x 023 mm. 



It is not necessary here to enter into the details of all the difterent kinds 

 of spicules and their variations which occur throughout the group, as this is 

 more fully dealt with under the various species, and the figures given there 

 will, moreover, give a much better idea of these than a lengthy description. 

 At the same time it might be well to define in a general way the various 

 types which liave been described. 



(«) Clubs. — Fig. 4 {a-fj) gives some idea of the variations of this type. 

 The general shape approaches that of the well-known Indian-club ; and the 

 most important characteristic is the fact that tlie warts or spines on the club- 

 porlion are all directed away from the shaft, the central part of which is 

 smootli. The spines do not arise peiiiendicularly (see also figs. 14, 23, 

 and 20). 



{h) Douhlc-chihs. — Fig. o («-?') shows two variations of this type. Tliey 

 have the shape of what arc usually known as dumb-bells. There is a distinct 

 median constriction which may be more or less well defined, and may vary 

 in length as well as in breadth (see figs. 27, 65, 75, and 113). The warts 

 may lio large or small, smooth, papillose, or very warty, but all arise perpen- 

 dicularly from the head (figs. 87 and 113), or they may be situated either 

 close together or wide apart ; and, according to which method occurs, the 

 head will be n?gular or irregular in outline (contrast figs. G3 and 65). The 

 head itself may be hemispherical or slightly conical (contrast figs. 54 and 65). 

 Fig. G {ii-d) shows characteristic variations of this type as seen in Juncella. 



(c) [Double-wheels or capsians. — Fig. 7 (« and b). This type consists of a 

 cylindrical shaft on which there arc two whorls or warts. The ends of this 

 shaft (the hubs) may be either almost smooth or markedly warty. 



(rf) Elowjnled douhlc-chibs.— This type may be derived from the typical 

 double-clubs, and merges gradually to another form which is sometimes 

 descrilied as double spindles, and this again may pass into the simple spiiuUc. 

 Fig. 8 a, b, and c show how these merge imperceptibly into one another. 

 In the cases we have describetl, however, these spicules are all of about the 

 same size, so that the distinction is made chiefiy ou the basis of the amount 

 of constriction visible and the proportionate length of the " head " to the 

 constriction (see figs. 63 and 65). 



(«■) Simple spindles. — In certain species, e.g. Nicclla dichotoma (see fig. 114), 

 there is a type of spicule which may be described as a simple spindle, and 

 which in size contrasts so strongly with the double-clubs that there are no 

 intermediate stages connecting the double-club with the spindle. (See also 

 fig. 118 of A'ieella nionilifvftne.) 



