378 PALAEONTOLOGY OF ILLINOIS. 



thin walls, and a variable number of delicate diaphragms. 

 Zooecia walls appreciably thickened in the mature region. Acan- 

 thopores small, more or less numerous, but nearly always re- 

 stricted to the angles of junction between the zooecia. Meso- 

 pores wanting. 

 Type: L. minima Ulrich. Range, Trenton to Hamilton. 



(?) DISCOTRYPA Ulrich. Zoaria consisting of very thin, free or 

 parasitic circular expansions. Surface with low broad monti- 

 cules, or even. Zooecia gradually decreasing in size from the 

 center of the monticules, very regular in their arrangement, 

 with thin walls, direct and generally hexagonal or rhomboidal 

 apertures. Mesopores and acanthopores entirely absent. 



Type : D. slogans Ulrich. Range, Cincinnati to Upper Helder- 

 berg. 



Family DIPLOTRYPID^E n. fam. 



Zoaria hemispheric, massive or ramose. Zooecia forming com- 

 paratively large tubes of which the walls are more or less flexu- 

 ous and mostly very thin. Mesopores and acanthopores pres- 

 ent or wanting. Diaphragms very thin, developed at rather ir- 

 regular intervals. No cystiphragms. 



DIPLOTRYPA Nicholson. Zoarium generally free and of hemi- 

 spheric or discoid shape; in other cases massive, or forming thin 

 crusts. Zooecia with thin walls and rather irregularly distributed 

 horizontal or oblique diaphragms. Acanthopores wanting in 

 the typical species, but present in some of the others. 



The genus as now understood, embraces at least three generi- 

 cally distinct sections, differing so much from each other as to 

 suggest affinities with as many different families. 



The typical section comprises, so far as known, only two species, 

 the European type of the genus, D. petropolitana, Pander, sp., 

 and D. westoni Ulrich, a species lately described from rocks in 

 Manitoba supposed to be of the age of the Cincinnati group. 



Another section, typified by D. infida Ulr., from the Trenton 

 shales of Minnesota, is closely related to Praxopora, while the 

 Niagara species, D. milleri Ulr., is just as closely allied to Callopo- 



