BEYOZOA. 303 



Monotrypa.] 



Diaphragms are few in the zooecial tubes, but in the mesopores, which are usually 

 constricted at the point of crossing, they are abundant. Here and there the zooecial 

 walls seem to diverge periodically so as to produce minute beaded tubuli. 



There is no parasitic bryozoan known to me from Lower Silurian rocks with 

 which this species could be confounded. Ramose Bryozoa coated with it might be 

 mistaken for certain varieties of Batostoma fertile, but the crusts are rarely complete 

 enough to render such a difficulty common. A greater superficial resemblance even 

 is sometimes presented to young examples of Pachydida foliata, a truly bifoliate 

 species, with very different internal structure and really so distinct that no one 

 ought ever to confuse them. 



Formation and locality. — Not uncommon in the lower and middle thirds of the Trenton shales at 

 Minneapolis and St. Paul, Minnesota. Occurs also in the "Lower Blue" at Beloit, Wisconsin. 



Genus MONOTRYPA, Nicholson. 



Monotrypa (part.), Nicholson, 1879, Pal. Tab. Oor., p. 293; 1881, Genus Monticulipora, pp. 102 and 



168; Ulbich, 1882, Jour. Gin. Soc. Nat. Hist., vol. v, p. 153; Foord, 

 1883, Contri. Micro-Pal. Can., p. 14. 



Monotrypa, Ulbich, 1890. Geol. Surv. 111., vol. viii, p. 379. 



Ptychonema, Hall, 1887. Pal. N. Y., vol. vi, p. xiii. 



Zoaria irregularly massive, discoid, or subglobose, apparently not divisible into 

 mature and immature regions. Zooecia comparatively large, prismatic, with very 

 thin, straight or transversely wrinkled walls; diaphragms complete, remotely placed 

 in the tubes. Both mesopores and acanthopores wanting. 



Type : M. undulata Nicholson, 



We are satisfied that thg position of this genus is near Diplotrypa (sensu strido) 

 and the simple section of the genus Batostoma. In the last we have only the ramose 

 habit of growth, and few and small mesopores -and acanthopores to distinguish it 

 from Monotrypa, the structure of the walls and the character of all the ^other points 

 being precisely the same in the two groups. In Diplotrypa diaphragms are perhaps 

 always more abundant, but in all other respects, excepting that the tapering prox- 

 imal ends of the zooecial tubes are closely tabulated like mesopores, the structure is 

 essentially the same as in Monotrypa. There is a largeness and a certain looseness 

 of arrangement that distinguishes the whole family Diplotrypidm from the Amplexo- 

 poridce, a family including (under Leptotrypa) a number of simple species agreeing 

 otherwise closely with Monotrypa. These species of Leptotrypa (e. g. L. filiosa d'Orb. 

 sp and L. petasiformis Nich. sp.) belong, -I am convinced, to a different line of devel- 

 opment than that of true Monotrypa. 



