171 



B. pala, B. angularis, and probably B. urceolaris, has a variety in 

 fact, several varieties with two posterior spines which are usually 

 symmetrically placed but not always symmetrically developed. The 

 form without posterior spines (var. dumorbicularis Skorikow) inter- 

 grades with these, and a series might be formed with complete 

 intergradations linking this in turn with var. rhenanus Lauterborn, 

 in which the spines are but slightly and often unequally developed. 

 From this we pass, by a slight elongation of the posterior spines, to 

 var. brevispinus Ehrbg., thence to the type in which- the spines as 

 figured by Rousselet ('97) are directed .posteriorly with but slight 

 curvature. From this we may pass toward variants in which the 

 symmetry is preserved, but the spines are much elongated and 

 curved outwardly. The anterior spines in such individuals are also 

 more elongated and exhibit a similar outward curvature (var. 

 melhemi Barrois and v. Daday). Extreme types of this curvature 

 sometimes occur (J5. falcatus Zach.). In another direction we find 

 the bilateral symmetry of the processes, both anterior and posterior, 

 to some extent lost as a result of differences in the curvature of the 

 spines (var. tuberculus Turner). There are also differences in the 

 surface markings of the lorica which have been utilized as specific 

 distinctions. Kertesz ('94) describes as B. granulatus a species 

 with a minutely pustulate surface, and Turner's B. tuberculus takes 

 its name from this same feature. It seems questionable, however, 

 if these surface markings are even of varietal value. Individuals 

 without spines, in which the transverse diameter is relatively large 

 (var. obesus Barrois and v. Daday) , are also found. 



In assorting the individuals belonging to this variable group I 

 have arranged them under the following heads: bakeri O. F. Mull., 

 bidentata Anderson (non bidentatus Kertesz), brevispinus Ehrbg., 

 cluniorbicularis Skor., melhemi Barrois and v. Daday, obesus Barrois 

 and v. Daday, rhenanus Lauterborn, and tuberculus Turner. The 

 number might have been increased. The individuals referred to 

 var. melhemi include many if not all of the long-spined specimens 

 such as Rousselet ('97) has referred to the type, the latter designa- 

 tion having been given to individuals intermediate between this and 

 brevispinus. The variety tuberculus includes the asymmetrical 

 individuals, regardless of the surface markings. I will now briefly 

 compare the seasonal distribution of these varieties and note 

 any peculiarities which mark them individually : 



