ON DIPLOGRAPTID^, LAPWORTH. 271 



opposite side of the hydrosoma. The alternating of the thecae, 

 therefore, is not only governed by the greater space attainable^ 

 but by the age and origin of the thecae as well. In certain 

 respects this law is also true for the scicula which may be 

 regarded, if desirable, as the primary theca. 



The growth lines meet on the outer edge of the hydrosoma in 

 such a way as to produce a zig-zag line (PI. II., fig. 7). This is 

 probably produced by the same cause as the marking of the lobes 

 of the scicula. Analogous to these, also, are the well-known paired 

 spines of the thecal apertures of certain diplograptids, which are 

 likewise an expression of the bilateral symmetry of the thecae. 



The partition wall between two adjoining or opposite thecae 

 is naturally double, and exhibits a slight thickening on the prox- 

 imal inner edge. 



The angle between the median line of the hydrosoma and the 

 double partitions of the thecae is greater in the distal portion^ 

 (25°-30°) than in the proximal, where it is occasionally zero. 



An examination of figures 8 and g shows that the scicula 

 originally lies outside of the hydrosoma, except for the loose 

 adherence of the thecae, being united to it only at the termina- 

 tion of the first theca. Nevertheless the earliest thecae are 

 indented on the dorsal side and partly enclosing the scicula, so 

 that it appears to lie in a depression of the outside of the hydro- 

 soma. The thecae extend more and more over the scicula until 

 the central space is nearly transformed into a tube encircling the 

 scicula, and when the fifth theca, i. e., the third on the same side 

 as the first, finally opens, the scicula then disappears into the 

 hydrosoma (PI. II., fig. 9). The place where the scicula comes in 

 contact with the perforated wall of the hydrosoma lies beneath 

 the boundary between the two parts of the scicula (PI. II., fig. 9). 



The Virgula. As the virgula has been observed to occur 

 within, and protrude from, both ends of the hydrosoma, it has 

 been naturally concluded that the virgula passed without inter- 

 ruption through the entire hydrosoma. This, however, is not 

 the case (PI. II., figs. 1-3). The origin of that portion of the 

 virgula which lies in the left wall of the proximal part of the 



