276 THE PALEONTOLOGY OF MINNESOTA. 



LCallopora. 



development slowly, with closely arranged diaphragms in the attenuate proximal 

 ends, and fewer or no diaphragms in the middle part of their length. In the per- 

 ipheral region these structures commonly increase again in number. Transverse 

 sections show that in the axial region the tubes are of two sizes, the larger ones 

 with six, seven, and most commonly eight sides, the smaller set four- or five-sided. 

 Type : C. elegantula Hall. 

 Thin sections of Callopora, vertical and transverse especially, exhibit a striking 

 uniformity of structure, so that it is often difficult to discriminate between those of 

 closely allied forms. Slight though generally recognizable peculiarities in the tab- 

 ulation of the zooecial tubes characterize the various species, but in a general way 

 the distribution of the diaphragms is essentially the same in all. The axial or prox- 

 imal end of the tubes always has diaphragms—sometimes only one or two (see 

 plate XXII, fig. 15) — while these structures may be wanting in the rest of the tube, 

 excepting one or two in the peripheral region. In other cases they will occur 

 throughout, but in every instance they "are less crowded in the central part of the 

 tube than in its inner and outer parts (see plate XXII, fig. 36). 



The twenty-five distinguishable forms now known to me of this, one of the best 

 characterized and most easily recognized genera of the Trepostomata, are distributed 

 through the various horizons intervening between the Birdseye and the top of the 

 Upper Silurian, one or more species being characteristic of every recognized geo- 

 logical division embraced in the interval. Nearly all the species again are to be 

 numbered among the common fossils, so that they may be said to be of the first 

 importance to the stratigrapher. The described species are distributed as follows : 

 In the Birdseye shales of Minnesota, C. angularis, C. incontroversa. and C. undulata; 

 in the overlying Trenton and Galena shales, C. dumalis, C. ampla, C. muUitahulata, 

 C. goodhuensis, C. crenulata, C. persimilis, and C. pulchella; in the four horizons of the 

 Cincinnati group going upward (1) C. nodulosa (Nicholson), C. sigillaroidea (Nich.), 

 (2) C. subplana Ulrich, C. dalei (Edwards and Haime), C. andrewsi (Nich.), C. ramosa 

 (d'Orb), (3) C. suhnodosa Ulrich, (4) C. n. sp.; in the Clinton, C. magnopora Foerste; 

 in the Niagara, C. elegantula Hall ; and in the Lower Helderberg, C. perelegans 

 Hall. 



The affinities of Callopora doubtlessly are with the Diplotrypidce and, but for the 

 perforated closures of the zocecia in Callopora, ' would recommend placing the genus 

 in that family. Aside from the closures, which, though they may really have existed, 

 have not been noticed in any member of that family, and a difference in the method 

 of growth, the zoaria of the one being ramose, of the other massive, Callopora a»d 

 Diplotrypa (in the restricted sense recently employed by me*) are very similar. It 



*Contrl. to' the Mlcro.-Pal. of the Camhro-Sil. Books of Oanada, pt. ii, p. 32, 1889. 



