Yol. 62.] IN THE CARBONIFEROUS ROCKS AT RUSH. 319 



LlTHOSTROTION CTATHOPHYLLOIDES, Sp. nOV. (PI. XXX, figS. 1, 1 a, 

 & 16.) 



Description. — Habit of growth: simple? (I have seen no 

 definite evidence of compound forms.) 



Form : cylindrical or elongate-conical. 



Epitheca: thin, annulatedby fine striae and broad, low swellings. 



Calyx : calyx-wall thick, and built up of numerous septa and of 

 closety-packed interseptal vesicles ; the inner slope of the wall is 

 steep. Calyx-floor radiated by the primary septa, which extend to 

 the centre. The central portion of the floor rises into a ridged tent, 

 up the slopes of which run the primary septa ; the ridge is crested 

 by the thickened columellarian plate. 



Horizontal section. — Peripheral area: often wanting, and 

 never continuous round the whole circumference ; where developed, 

 it is thin and purely vesicular. 



External area: broad, and closely radiated by the two cycles of 

 septa; there are eight to ten rows of vesicles in the interseptal spaces. 



Medial area: narrow, and almost clear of interseptal vesicles. 

 The primary septa cross the area, but the secondary septa only 

 project for a short distance into its outer margin. 



Central area: completely radiated by the primary septa, which 

 are crossed by about seven tabular intersections ; the outer inter- 

 sections are more closely approximated, so as to produce a con- 

 spicuous ring of denser structure. The columella is an elongated 

 central plate. 



Septa: not appreciably thickened in any part of their length. 

 The primary septa are distinctly flexuous in the central area. 



Measurements. — The central type has a diameter of about 

 21 millimetres, and has about forty-four primary septa ; the relative 

 proportions of the areas are best appreciated from the figure of a 

 horizontal section (PI. XXX, fig. 1 a). 



Range and variation. — (1) The central type is abundant in 

 the Megastoma-ltieds ; the specimens named Lithostrotion cf. cyatlio- 

 phylloides from the CyatJicLvonia-Beds have only been examined 

 in rough fracture. 



(2) A mutation occurs somewhat sparingly in the Curkeen 

 Limestone. This mutation is easily distinguished by its more 

 numerous septa (about fifty) and by the less open character of the 

 medial area, due to the greater elongation of the secondary septa 

 (see PL XXX, fig. 1 b). 



Comparisons. — (1) With other Lithostrotions : L. affine agrees 

 in the considerable development of vesicles in the external area ; 

 but differs in the shortness of the septa, which do not extend to 

 the centre. The large Xematophylla (that is, the basaltiform 

 Lithostrotions) agree in : 



(a) The extension of the septa to the centre ; and (b) the conical tabulae 

 of the central area and the columella. 



They differ, however, in the smaller number of their septa, as 

 well as in their manner of growth. 



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