6 Dr. Ci. Lindstiuiii on the Acuities 



tissue, resembling a ca-nencliyma, and eonsisting of smaller 

 circular or polygonal tubes. The walls of the zooecia are 

 solid, -without any perforations, and interiorly quite smooth 

 and destitute of jirojecting ridges or septa. The tabulae are 

 very irregular in the large tubes, being oblique or deeply sunk 

 at the walls ; in the narrower tubes they are dense and regular. 

 The large zooecia are clustered in groups at tolerably regular 

 intervals, each group of six or eiglit members. In Upper- 

 Silurian specimens they very seldom project above the surface, 

 and do not form the strange monticules wliich are so common 

 on the surface of the Russian Lower-Silurian specimens. I 

 suppose that these clusters are continuations from the original 

 and larger zoacia, which were budded out round the smooth 

 centra when the colony was in its Ceramopora stage. In 

 some there is seen a sort of " reversion," the zooecia on the 

 surfaceof il/o«<ic?///)?orahaving again assumed the unmistakable 

 characters of a Bryozoon, becoming oblique, and radiating as 

 in a Ceramopora. Longitudinal sections, however, demonstrate 

 that there is a direct continuation from the tubes of the Monti- 

 culipora into those of the Ceramopora, or that the former again 

 have changed into the latter. 



A more common and more protean Monticulipora is that 

 which Hall described as Trematopora ostiolata (Pal. N. Y. 

 vol. ii. p. 152, pi. 40. fig. 5), and which I consider to be 

 identical with M. papillata, M^Coy (Edw. & Haime, Brit. 

 Foss. Cor. p. 266, pi. 62. figs. 4, 4 «), with Thecostegites hemi- 

 sphcericiis (Ferd. Homer, ' Tennessee,' p. 25, })1. ii. tigs. 3, 3a), 

 andwith Htxctopora malmoensis, Kjerulf (Veiviser,p. 21, fig. 29). 

 All these are only different stages of growth of the same species, 

 viz. Monticulipora ostiolata, tlie fully developed form belonging 

 to this genus. The DiscoporeJla stage, the initial one, con- 

 sists of a thin crust covered with small tubular zooecia, varying 

 in form, Avith oval or crescentic mouths, or having the sides 

 faintly indented, with a short spine at each indentation. Inter- 

 stitial ribs are also present. The smallest colony I have seen 

 is 3 millims. in diameter ; and, as in the Discoporellai in 

 general, the centre is smooth and concave, without zooecia, 

 but sun'ounded by cells radiating in all directions. As this 

 primitive colony ahvays s])reads as a thin membrane over the 

 object on which it is fixed, its shape depends on the shape of 

 its basis ; and in consequence the polyparium is discoidal, glo- 

 bular, or branching ; rarely it is semigiobular, on its own free 

 basis. From this Discopjorella .stage it passes into what may 

 aptly be called the Fistulipora stage. The genus Fistulipora 

 is, indeed, chiefly made up of Silurian and Devonian Bryozoa. 

 The cells are now elevated, some being angular, the walls 

 being bent inwards in 3-4 (or sometimes only 1-2) folds, which 



