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€r. W. SHETJBSOLE EEETHEE NOTES ON 



two lines transversely. Fenestrules gradually increasing in size 

 with the growth. Pores small, round, more than their own diameter 

 apart, placed on the outer face of the polyzoarium. At the base 

 three pores within the length of a fenestrule ; in the upper portion 

 four in the same space. 



Ohs. There are many points of considerable interest attaching to 

 this species. In shape and appearance it is an enlarged type of 

 the Silurian species generally, and particularly of my Fenestella 

 lineata. It is the only representative of the old type which has 

 survived to Carboniferous times. In addition to its shape, it has 

 most of the features of its prototype, viz. interstices seldom bifurca- 

 ting, proceeding in parallel lines, and pores on outer face of polyzoary. 

 Its early growth, from a minute point, was a tapering, often curved 

 root-base, from which grew a hollow and gradually widening cone, 

 which ultimately expanded in slightly folded and lobed outlines 

 around the aperture. To secure in position so fragile a structure, 

 numerous solid rootlets grew from various points of the base, and 

 attached themselves to surrounding objects. Being somewhat cylin- 

 drical, it did not need to make the usual amount of lateral growth 

 by bifurcation, as in the ordinary open type of Carboniferous 

 Fenestellce. Owing to this comparative absence of bifurcation in 

 the insterstice, its lines are singularly regular, forming a series of 

 parallel rows. Further, its lateral growth was obtained by a gradual 

 enlargement of all parts of the structure, proceeding from the base 

 upwards — so much so that while at the base four fenestrules may be 

 counted in one line, at the distance of two inches there are only two 

 in the same space, thus doubling the circumference of the polyzoary 

 without the aid of division of the interstice. This enlargement in 

 the growth is more or less persistent throughout the polyzoarium, 

 but not to the same extent, and furnishes at once a key to the 

 synonyms. Accompanying the increased growth, an additional 

 pore-cell may be noticed between the dessepiments. Thus the 

 difference between the base and the upper portion, both as to size 

 and shape, is of a very marked character, and quite accounts for the 

 several species into which this one has been divided. Phillips was 

 the earliest worker at it ; he named the extreme base Fenestella 

 membranacea, the upper and enlarged growth Fenestella flabellaia, 

 and the more delicately formed and intermediate portion Fenestella 

 tenuijila. 



I have alluded to the fact that in the more cylindrical portion of 

 F. membranacea bifurcation of the insterstice was arrested in part. 

 At times a variety of circumstances may have hindered the longi- 

 tudinal extension of the polyzoary ; then we find a rapid bifurcation 

 which gives a globose outline to the expansion, and the form is then 

 the F. hemisplierica of M'Coy, while all its other details as to the 

 pores, interstices, &c. clearly point to its identity with F. membra- 

 nacea, Phill. 



The only species likely to be confounded with the foregoing is the 

 F. nodulosa of Phillips. The square form of the fenestrule in the 

 latter will at once indicate its character. 



