172 THE GENUS MONTICULIPORA. 



hemispherical or spheroidal shape (fig. 33, a). I have figured 

 thill sections of both for comparison. 



Tangential sections of both the Trenton Limestone and 

 Hudson River Group examples of M. 7indulata (figs. 32, 33, b) 

 show the corallites to be strikingly thin-walled and markedly 

 angular, while, except for the occasional presence of a cluster of 

 somewhat extra-sized tubes, their dimensions are very uniform. 

 Small corallites are present, not unfrequently, at the angles 

 of junction of the large tubes ; but they are obviously young 

 tubes, and do not form part of a series of special corallites. 

 That this view is correct is shown by their inconstant occur- 

 rence, but is still more conclusively proved by vertical sections 

 (fig. 32, c). These show that all the corallites — those forming 

 the clusters as well as those composing the mass of the colony 

 — are precisely similar in their structure, and are not divisible 

 into a series with remote and one with crowded tabulae. All 

 alike have thin, flexuous, often closely undulated walls, and 

 in all alike the tabulae are delicate horizontal plates, situated 

 at distances of from a quarter of a line to nearly a line. In 

 fractured surfaces the tubes separate cleanly from one another, 

 and their faces are seen to be crossed by numerous delicate 

 transverse striae, corresponding with the undulations of their 

 walls. In all the specimens I have examined there is, also, an 

 evident periodicity of growth, tabulae being periodically de- 

 veloped at corresponding levels in all the tubes, so that the 

 entire corallum breaks up into concentric layers. 



As before remarked, I see no reason to doubt that the 

 massive examples of ]\I. inidiilata from the Trenton Limestone 

 are specifically identical with the smaller rounded masses which 

 occur in the Hudson River Group; at the same time that I re- 

 gard the former as the type of the species. The chief differ- 

 ences between these two forms may be advantageously added 

 here, their chief points of agreement having been previously 

 noticed. The typical exahiple from the Trenton Limestone 

 (fig. 32, a) is a large undulated and folded mass, with a maxi- 

 mum diameter of about four inches, and a heifrht of about 



