320 TABULATE CORALS. 



Sub-gemis Monotrypa, Nicholson, 1879. 



I propose the name of Mo7iotrypa for a few forms of Mo7iti- 

 adipora which agree with those that I have called Diplotrypa 

 in the fact that the corallites are thin-walled throughout their 

 entire extent, and are so amalgamated in contiguous tubes that 

 their originally double structure is not recognisable. On the 

 other hand, they differ from Diplotrypa in the fact that the 

 closely-tabulate small corallites of the latter are now totally 

 absent, the colony consisting of subequaL tubes which are uni- 

 formly and throughout provided with remote and complete 

 tabulae. The only sign of dimorphism, in fact, which can be 

 detected in the corallum of Monotrypa is the constant existence 

 of special clusters (" monticules ") of corallites, which are de- 

 cidedly larger than the average tubes, though quite like these 

 in internal structure. 



There are several respects in which this section of the genus 

 Monticulipora must be considered as not wholly satisfactory, at 

 the same time that the composition of the corallum out of tubes 

 which are identical in internal structure, and which differ very 

 little in point of size, precludes our associating the forms in 

 question with any of those we have been previously considering. 

 In some points Monotrypa makes an approach to Chcstetes 

 proper, but the much greater thinness of the walls of the coral- 

 lites, and the constant presence of clusters of large tubes, suffi- 

 ciently distinguish the species of the former from those of the 

 latter. The only two forms which I at present know as com- 

 bining all the features here ascribed to Monotrypa are those 

 which I shall describe immediately under the name of M. un- 

 dulata and M. Winteri. Typical examples of the M. pidchella, 

 E. and H., from the Upper Silurian of Britain, show, however, 

 all the general characters of Monotrypa (see PI. XIV., figs. 

 6, 6 a), except that the walls of the corallites become decidedly 

 thickened in approaching the surface, and the boundary-lines 

 between contiguous tubes are usually clearly marked. (I have 



