364 PALAEONTOLOGY OP ILLINOIS. 



representatives also prevail in this case. Namely, the zocecia 

 are more tubular than those of the Vincularids, while the 

 secondary deposit which gave rise to the "vestibule" of the 

 CRYPTOSTOMATA is wanting in them. In other respects the closest 

 agreement can be shown. On the other hand the RHABDOME- 

 SONTID.E are intimately connected with the BATOSTOMELLID.E of 

 the TREPOSTOMATA, and it is sometimes very difficult to draw 

 the line between Rhombopora and the genera of that family. 

 The genus Rhombopora also embraces species that compare 

 very favorably with Streblotrypa. In fact the genus contains 

 some very diverse yet intimately related forms of which each 

 points in widely different directions. 



In Rhabdomeson Y. and Y., the zocecia are arranged round an 

 axial tube precisely the same as in the cretaceous Siphonella of 

 Hagenow. Such an axis does not exist in Rhombopora nor 

 have I yet seen an American species of this family in which it 

 is present. The structure of the zooecia is about as follows: 

 Their primitive portion is always tubular and enclosed by ex- 

 ceedingly thin walls. Their proximal ends may be attached to 

 an axial tube (Rhabdomeson}, or they may originate along an 

 axial line around which they are wedged in a radial manner 

 (Rhombopora incrassata, PI. LXX, fig. 12d) or they are pro- 

 duced by germination throughout the axial region (R. suban- 

 nulata, PI. XLV, and R. tabulata and R. minor, PL LXX). 

 Here too the longer tubes may be intersected by one or more 

 complete diaphragms. These structures must not be confounded 

 with the hemisepta which are usually present at the point of 

 outward bending and thickening of the walls. As in PtUodidyn, 

 two or more pairs of hemisepta may be developed in old exam- 

 ples, but in most cases the inferior hemisepta appear to be 

 absent, (PI. XLV, fig. Ih, and PI. LXXI, fig. 3c). The aper- 

 ture is circular or elliptical and, with only few exceptions, sur- 

 rounded by a sloping area, typically rhombic or hexagonal in 

 outline, causing the inter-apertural space to be ridge-shaped. 

 The summit of the ridge carries from one to three series of 

 minute granules, while an acanthopore is commonly found at 

 the angles of junction. 



As already intimated the family foreshadows the VINCULARID/E. 

 Among Palaeozoic Bryozoa the BATOSTOMELLID^E offer the most 



