METAZOA COELENTERA. 91 



the larger end and this forms a protecting cup or theca. 

 While this is the rule, there are genera in which the bud 

 arises from the middle portion of the sicula and from the 

 smaller end. As a general thing the sicula is retained 

 unchanged in form by the mature animal, but in a few 

 species it is absorbed or becomes obsolete in old age. 



The group of Monograptidae is represented by Mono- 

 graptus (No. 101), which has a single series of cups or 

 thecae on one side and at their base a well developed 

 virgula. 



Wiman l has made a study of the Diplograptidae 

 which are represented in the Collection by Diplograptus 

 (No. 102). He finds that these forms arise in the same 

 way as the Monograptidae, and it seems probable that 

 they are the specialized descendants of the last named 

 group. The sicula of Diplograptus (PI. 103, fig. i, 

 young, dorsal view; fig. 2, adult, front view) consists of 

 two parts : the proximal portion marked diagonally ; the 

 distal, longitudinally (seen in fig. i). The sicula is open 

 at its base, and at one side is the rod or virgula. This 

 sicula gives forth one bud only, which does not develop 

 into a canal as heretofore supposed, but into a cup or 

 theca. Fig. 3 is a front view of the first theca budded 

 from the sicula, and fig. 4 is a dorsal view of the same. 



The circular perforation in fig. 3 marks its origin from 

 the sicula. The theca grows downward, then outward. 

 Fig. 5 is the first theca with three spines, two of which 

 are united by a thin skin. The theca is seen to have 

 grown outward and upward. In time this theca buds and 

 the second theca grows around to the opposite side. This 

 process is repeated, the second giving rise to the third, the 

 third to the fourth, so that the statement can be made that 

 each theca comes from the next more proximally situated 

 theca of the opposite side and not from a canal. Fig. 6 



1 Journ. of Geol., II, no. 3, Apr.-May, 1894, p. 267. See also 

 Holm, Geol. Mag., London, Decade IV, II, 1895. 



