290 Dr. H. A. Nicholson’s Contributions 
S. Howsii, var. arctica, is a larger and more massive form 
than the normal examples of the species, but its general mode 
of growth is the same, and there is no essential difference as 
regards their general structure. Tabule are, however, less 
abundantly developed in the peripheral region of the corallum 
(Pl. X. fig. 11), and are therefore separated by wider inter- 
vals. On the other hand, there is a specially extensive 
development of the curious thick-walled tubes which I have 
described as occurring in the normal form of 8. Hows?z. 
These remarkable structures are best observed in tangential 
sections, whether these traverse the corallum at the level of 
the unthickened segments of the corallites (fig. 2, A) or at 
that of the thickened nodes (fig. 2, B). In longitudinal 
A. Part of a tangential section of Stenopora Howsti, var. arctica, 
taken at the level of the unthickened segments of the tubes, enlarged 
twenty-four times. B. Another part of the same section, taken at the 
level of the thickened parts of the tubes, similarly enlarged. 
sections (Pl. X. fig. 11) the same structures appear as tubes 
running in the thickness of the walls of the ordinary corallites. 
While these singular tubes are remarkably abundant, the 
interstitial tubuli which are such astriking feature in thenormal 
form are here somewhat diminished in number. The chief 
point, however, by which 8. Hows?’, var. arctica, is distin- 
guished from the type of the species is that the walls of the 
corallites are decidedly thicker and the visceral chambers 
more contracted than in the latter, while each visceral chamber 
is surrounded by a ring-like investment of fibrous sclerenchyma 
(fig. 2, B). 
formation and Locality. 'The type of the species occurs in 
