The Annals and magazine of natural history; zoology, botany, and geology . They exhibit, however,certain subordinate points of distinction from the British types,and I think it best therefore to mark the existence of these bythe addition of a varietal name. 290 Dr. H. A. Nicholsons Contributions 8. Howsii, var. arctica, is a larger and more massive formthan the normal examples of the species, bnt its general modeof growth is the same, and there is no essential difference asregards their general structure. Tabula? are, however, lessabundantly developed in the peripheral region of the corallum(P
The Annals and magazine of natural history; zoology, botany, and geology . They exhibit, however,certain subordinate points of distinction from the British types,and I think it best therefore to mark the existence of these bythe addition of a varietal name. 290 Dr. H. A. Nicholsons Contributions 8. Howsii, var. arctica, is a larger and more massive formthan the normal examples of the species, bnt its general modeof growth is the same, and there is no essential difference asregards their general structure. Tabula? are, however, lessabundantly developed in the peripheral region of the corallum(PI. X. fig. 11), and are therefore separated by wider inter-vals. On the other hand, there is a specially extensivedevelopment of the curious thick-walled tubes which I havedescribed as occurring in the normal form of 8. remarkable structures are best observed in tangentialsections, whether these traverse the corallum at the level ofthe unthickened segments of the corallites (fig. 2, A) or atthat of the thickened nodes (fig. 2, B). In longitudinal Fiar. A. Part of a tangential section of Stenopora Howsii, var. arctica, taken at the level of the unthickened segments of the tubes, enlargedtwenty-four times. B. Another part of the same section, taken at thelevel of the thickened parts of the tubes, similarly enlarged. sections (PI. X. fig. 11) the same structures appear as tubesrunning in the thickness of the walls of the ordinary corallites. While these singular tubes are remarkably abundant, theinterstitial tubuli which are such a striking feature in the normalform are here somewhat diminished in number. The chiefpoint, however, by which S. Howsii, var. arctica, is distin-guished from the type of the species is that the walls of thecorallites are decidedly thicker and the visceral chambersmore contracted than in the latter, while each visceral chamberis surrounded by a ring-like investment of fibrous sclerenchyma(,B). Formation and Locality. The type of the
Size: 2012px × 1242px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookce, booksubjectbotany, booksubjectgeology, booksubjectzoology
