418 Dr. H. A. Nicholson on the Minute 



their central portions we have the longitudinally divided 

 corallites of the axis of the branch, which are here provided 

 with very thin walls, and are nearly vertical in direction. 

 Their cavities in this region are also intersected by but a very 

 small number of " tabulae," though these structures are pre- 

 sent in small amount. In the outer part of longitudinal 

 sections (as in the peripheral portions of transverse sections) 

 we can study the characters of the tubes in their external 

 thickened region, where they have become bent outwards on 

 their way towards the surface (fig. 4, D). In this part of 

 their course the walls of the corallites have become consider- 

 ably thickened, though there is never any difficulty in recog- 

 nizing the line of demarcation between the proper investment 

 of any one tube and that of its neighbours. No indications are 

 visible of any canals or pores placing the cavities of contigu- 

 ous tubes in communication ; but the visceral chambers are 

 traversed by numerous complete and well-developed trans- 

 verse partitions, or "tabula?," which continue to be developed 

 till close upon the actual mouths of the tubes. Between the 

 larger corallites we can now also readily distinguish the 

 smaller interstitial corallites, which can be at once distin- 

 guished, not only by their more diminutive size, but likewise 

 by the distinct structural character that they are provided 

 with more numerous and closely set " tabulae " than is the 

 case with the normal tubes. 



Monticulipora pulchella* , Edw. & Haime. 



As there are considerable differences (differences of at least 

 subgeneric, possibly of generic, value) between the various forms 

 usually included by palaeontologists under the common name of 

 Monticulipora, I have thought it advisable to give here a 

 short description of the structure of a type of the genus 

 markedly distinct from M. Jamesi, Nich. ; and for this purpose 

 I have selected the well-known M. pulchetta, E. & H., of 

 the Upper Silurian deposits of Britain. 



The typical M. pulchella, E. & H., as regards its shape 

 and general appearance (fig. 3, C), is very similar to Hetero- 

 pora neozelantca, Busk, except that the mouths of the tubes 



* It may be noted here that the form which I have described from 

 the Lower Silurian rocks of North America under the name of Chcetetes 

 pukhellus (Pal. Ohio, vol. ii. p. 195, and Quart. Journ. Geol. Soc. vol. xxx. 

 p. 503) is not, as I have now ascertained, the same as Monticulipora pul- 

 chella, E. &; H. It very closely resembles the true M. pulchella iu external 

 characters, and especially in the possession of clusters of large tubes 

 interspersed at intervals among the average corallites of the corallum ; 

 but I find its internal structure to be such as to entirely separate it 

 from the British species. 



