Zoology.'] NATURAL HISTORY OF VICTORIA. [Polyzoa. 



base supported by two calcareous processes rising vertically from 

 the thickened posterior margin of the mouth. 



It is at once distinguished from the last by its much larger size, 

 more massive appearance, the usually tricellate internodes, and the 

 different sculpture on the back. 



Explanation of Figures. 



Plate 108. — Fig. 1, portion of branch, natural size. Fig. lor, front view of two internodes, 

 magnified. Fig. \b, portion of branch in outline, to show internodes of two and of three cells. 

 Fig. \c, back of single internode. 



The genus Calpidium was founded by Busk to contain 

 C. ornatum, the character being that "it is distinguishable by 

 the anomalous circumstance that each cell is furnished with two 

 or more, usually with three distinct keyhole-shaped mouths, and is 

 doubtless inhabited by three distinct individuals." The genus is a 

 very marked and natural one, but the real distinction is not in the 

 number but in the structure of the mouths. The mouth is lofty, 

 contracted about the junction of the middle with the lower third ; 

 above this the margin is very prominent, and projects as a hood or 

 collar, slightly hollowed in the centre above, from which point (in 

 the median or primary cells) a narrow ridge runs upwards. Each 

 internode consists of a primary cell, either single, or with an 

 additional cell added on one or both sides. The cells of a series, 

 except at a bifurcation, always arise from the summit of a primary 

 cell. At a bifurcation in C. ornatum one branch springs from the 

 primary cell, the other from one of the laterals, Avhich has also a 

 vertical ridge above the aperture ; while in C. ponderosum both 

 cells at a bifurcation have a similar ridge. In continuous series, 

 where the internodes are double-celled, the secondary cell seems 

 to be always developed on the same side. At each upper angle of 

 the internode, whether consisting of 1, 2, or o cells, there is a 

 considerable avicularium . 



Catenicella aurita and C. geminata have the mouth of a similar 

 shape, but destitute of the raised margin, They ought, I think, 

 to be separated as a distinct genus. 



[ 34] 



