216 THE PALEONTOLOGY OF MINNESOTA. 



LMontioullporldse. 



On plate XIX the figures up to No. 19 give an idea of the beginning of the line 

 of development referred to in the latter part of the paragraph preceding the last. In 

 Homotrypa minne'sotensis we have what must still be regarded as a true species of the 

 geuus.* Even here, as shown in fig. 3, a tendency to interpolate mesopore-like 

 interspaces is occasionally manifest. In H. exilis this tendency is more strikingly 

 expressed and permanent, (see figs 13-16), while in H. separata it has progressed to 

 such an extent that the zooecia are rounded in shape instead of angular. These 

 three forms all occur in the lower third of the Trenton shales. In the middle third 

 we find a fourth stage in the development toward the Galena shales species provis- 

 ionally called Homotrypella, ovata. This fourth stage has been named Homotrypa (?) 

 intercalaris and, as may be seen from the cut on a succeeding page, its characters are 

 very nearly intermediate between those of the species separata and ovafa. 



Difficulties like these were encountered all through my work on the Minnesota 

 Bryozoa, and now as it nears completion I realize, probably better than any one else 

 can, that the result, despite my utmost efibrts, is not final nor even entirely satis- 

 factory in some parts. Had I followed my inclination and adopted a genealogical 

 arrangement throughout it might have been better, because I believe it would have 

 proven more permanent. But the criticism sometimes made that individual peculi- 

 arities are magnified into specific, and specific into generic, seemed to indicate a state 

 of knowledge not sufiiciently advanced for the proper appreciation of air the innova- 

 tions that it would have been necessary to enter into. Perhaps I ought not to have 

 noticed criticisms which, like these, originate in ignorance of the fact that characters 

 preserved in hundreds of specimens are not to be viewed as "individual." But I 

 have done so, and now must hope that the use of the interrogation point in cases of 

 the kind discussed may be deemed sufficient for immediate needs. 



To prohibit repetition as much as possible I shall, in the course of the following 

 pages, frequently refer the student to the preceding paragraphs. 



Family MONTICITLIPORIDJI, Nicholson, emend. Ulrich. 



This family is strongly represented in the Lower Silurian rocks of Minnesota, there 

 being here one or more species of every genus included in the family except Perono- 

 pora, Nicholson. The individuals of the species too are generally abundant, especially 

 those of Prasopora and Homotrypa, so that they constitute no inconsiderable part of 

 the collections from the various horizons. 



*H, subramoaa, illustrated on the same plate, is more nearly like the prevailing Hudson Eiver group types of the 

 genus. 



