338 PALEONTOLOGY OF ILLINOIS. 



somewhat shadowy and certainly much less sharp than we find 

 it to have been during the early periods of their existence. So 

 little is known of the internal structure of Mesozoic CYCLOSTO- 

 MATA and TREPOSTOMATA that it is not now possible to dis- 

 tinguish satisfactorily between them. It would be presumptive 

 on my part to attempt it. In a general way. however, it may 

 be said that the Mesozoic types which are provided with meso- 

 pores, such as the majority of the genera referred by d'Orbigny 

 to his families, CLAUSED js, CAVEID.E, CAVID.E and CRESCISID.E, are 

 not true CYCLOSTOMATA but rather TREPOSTOMATA. 



The Palaeozoic genera I have grouped under the nine families, 

 MONTICULIPORID.E, HETEROTRYPID^E, CALLOPORID^E, TREMATO- 

 PORID.E, BATOSTOMELLULE, AMPLEXOPORID.E, DIPLOTRYPID.E, CER- 

 AMOPORID.E, and FISTULIPORHLE. The first may be considered 

 as the most typical, as the forms which constitute the family 

 contain all the more characteristic features of the sub-order. 



(1) MONTICULIPORHLE: The zoarium in this group comprising 

 the genera Monticulipora, Peronopom, Atactoporelki , Praso- 

 pora, Homotrypa and Homotrypella, may be massive, discoid, 

 lamellar or incrusting, while the bifoliate and ramose forms are 

 also frequent. The zocecia are prismatic, rounded, or irregu- 

 larly petaloid. with comparatively thin walls, having a peculiar 

 granular structure. The mesopores vary greatly in number, 

 being sometimes apparently absent or very few, at other times 

 very numerous. Occasionally they are obscured by a secondary 

 deposit of sclerenchyma. Zooecia with cystiphragms, the meso- 

 pores with horizontal diaphragms. The acanthopores are nearly 

 always small, and generally numerous. 



The family as above restricted contains a large number of 

 mainly Lower Silurian Bryozoa, whose principal character, the 

 cystiphragms, has, so far, proved an almost infallible distin- 

 guishing mark. In the genus Monticulipora (e. g. M. lamellosa 

 and M. winchelli) these structures are often modified so that 

 they might be mistaken for simple, oblique or slightly curved 

 diaphragms. Experience, however, will soon dispel the illusion. 

 The genera of the family are separated from each other mainly 

 by constant zoarial peculiarities, though each also has its own 



distinguishing zooecial features. 



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