219 



only to a slight extent takes part in its formation in most species of the genus 

 Scuticella. When the cryptocyst lamina expands on the inner surface of the frontal 

 wall the ribs grow at the same time from the bilaminar, uncalcified marginal 

 portion, in which the cryptocyst and the gymnocyst meet, and they must there- 

 fore themselves be considered bilaipiinar, although the two layers in most cases 

 will be coalesced into one solid rib. In some cases e. g. in Scuticella margaritacea, 

 var. connectens (PI. XX, figs. 3 b, 3 c) these spines, between which there is left a 

 part of the original sinus, remain uncalcified at the end, and this may be seen 

 most plainly in the gonozooecium. 



While the above mentioned cryptocyst lamina is originally only continuous 

 with the margin of the primary aperture and is otherwise free (PI. XI, fig. 1 n cpl.), 

 it gradually with growth coalesces with the inner surface of the frontal wall to 

 a varying extent and in different ways; it is for instance distinctly seen in Costi- 

 cella hastata (PL XII, figs. 1 a — 1 d, see explanation of plates) that the hollow in 

 the quadrangular, plate-like spines rising from the sternal sinus, is continued 

 bej'ond their starting point up to the round fenestrse in the sternal area. The 

 fact is that the regions, separating these hollow spaces from each other, are 

 formed by a fusion or soldering of the cryptocyst lamina with the inner surface 

 of the gymnocyst. 



With the exception of a few species (Calpidium ornatum, Pterocella gemelld) 

 in which every internode consists of two or three zooecia, we find in the others 

 internodes with one and internodes with two zooecia alternating more or less 

 regularly with each other, but we may in this respect distinguish between two 

 cases, which do not however show any important difl"erences, as both may appear 

 within the same genus. In one case a bizooecial internode always takes its origin 

 from a unizooecial one, while here and there a series of successive single zooecia 

 may spring from the daughter-zooecium of a bizooecial joint. This is the case 

 e. g. in Scuticella plagistoma, Sc. intermedia, Sc. lorica, So. Wilsoni, Sc. amphora, 

 Sc. maculata, Cribricella rufa, Crib, cribraria, Catenaria perforata. Cat. elegans and 

 Cat. formosa. In the other case one bizooecial segment may on the contrary often 

 follow another, which may be repeated at least three times, while at the same 

 time an alternation of uni- and bizooecial internodes may be seen in the neighbour- 

 ing branches. This may be seen in Scuticella ventricosa, Sc. margaritacea, Ptero- 

 cella alata, Claviporella geminata, Catenaria Buski, Cat. fusca. Cat. taurina and 

 Strophipora Harveyi. 



