284 THE PALEONTOLOGY OF MINNESOTA. 



[Oallopora pulchella var. preslmills. 



smaller zocecia. Excepting the following variety there is no Minnesota species of 

 Callopom known that is likely to be confounded with C. pulchella, but care is required 

 in separating it from the associated Homotrypa tuberculata. 



Formation and locality. Common in the upper third of the Trenton shales at St. Paul and near 

 Cannon Falls, Minnesota. 



Mus. Reg. Nos. 8033, 8114. 



CALLOPORA PULCHELLA, var. PEESIMILIS, n. var. 



PLATE XXII, FIGS. 13-17. 



This variety differs from the typical form of the species in having more prom- 

 inent tubercles, thinner zooscial walls (causing the apertures to be correspondingly 

 larger), more readily distinguished and, as a rule, more numerous mesopores, even 

 fewer diaphragms, and in the greater size of the tubes in the axial region. The 

 growth of the zoarium and the number of zoaacial apertures in 3 mm. (thirteen or 

 fourteen) is the same. Indeed, the points of difference even are not constant. We 

 cannot, therefore, doubt the propriety of referring the form to C. pulchella. On the 

 other hand it is impossible to distinguish var. persimilis, by external comparison of 

 fragmentary material alone, from the later C. ramosa d'Orbigny, one of the most 

 common Bryozoa of the Cincinnati rocks. It is true the zoaria of the Trenton form 

 never grew to such a size nor do the branches inosculate as in the Cincinnati species, 

 but such differences the student will find of little practical use when, as usual, frag- 

 ments are to be identified. Luckily, the interior furnishes us with a guide in the 

 relative number of diaphragms, these being much more abundantly developed in the 

 Cincinnati species than in the Trenton variety. For further remarks relating to 

 this subject see ante. p. 215. 



Formation and locality. Associated with Phylloporina corticosa, Trigonodictya conciliatrix and 

 Prasopora conoidea, all characteristic species of the upper third of the Trenton shales, at Poe's farm, 

 near Cannon Falls, Minnesota. 



Mus. Beg. No. 8115. 



CALLOPORA CRENULATA, n. sp. 



PLATE XXII, FIGS. 18-23. 



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Zoaria forming rather large, compactly interwoven, bushy masses, consisting of 

 strong branches that divide and inosculate most irregularly and frequently; average 

 thickness of branches 7 or 8 mm., but some may be over 10 mm., and as many only 

 4 or 5 mm. Surface generally with rounded monticules, occupied by clusters of 

 zooecia larger than the average, and by greater or less aggregations of small meso- 



