PROBLEM OF THE MONTICULIPOROIDEA I 5 



area, as proved by sections, is composed of large, angular meso- 

 pores, and the ridges exclusively of rounded autocells, these 

 tending to arrange in two rows, with the walls between the rows 

 a little raised and straightened. The space between monticules 

 is small and occupied by mesopores around a few single auto- 

 cells, and groups of two, three, four, etc., cells or incipient 

 ridges. In longitudinal sections the close tabulae only distin- 

 guish the mesopores at first, but they soon become vesiculose. 



This description is taken from specimens from the "Trenton 

 shales" of the Northwest. The species is very rare as com- 

 pared to the acrogene ones generally called Constellaria. 



DISCUSSION OF TREPOSTOMATA 



A few species suffice to illustrate the general characters of 

 Trepostomata, and the further detail may be explained by them. 

 In this manner of beginning with a few representative species, 

 proceeding thence to the study of the several characters recurring 

 in the whole group, the perplexing taxonomic definitions may 

 be obviated. 



The growth habit or zoarial form is fairly constant in the 

 species, but very various beyond that taxonomic limit. The 

 approximately hemispheric zoarium, with its cells radiating and 

 multiplying with growth from an initial point, may be taken as 

 the central or composite or primitive type. Next, species with 

 an established tendency to grow fastest around the base develop 

 the flattened massive type. Others, more perigene in degrees, 

 connect with the laminate or encrusting, in which perigene 

 growth is near its practicable extreme. 



On the other hand, the hemispheric form, by increased 

 acrogene growth tendency, becomes the digitate, and finally 

 dendroid branched. But, as a rule, the acrogene zoarium has a 

 perigene basal expansion, and every degree of form might be 

 pointed out from the conical (PI. A, Fig. 7), in which moderate 

 acrogene and perigene combine, to the strongly acrogene form, 

 with more or less extremely perigene basal (Fig. 5). As to the 

 basal expansion, it may be massive, laminate to encrusting. It 



