152 ME. F. H. C0WPER REED OX THE EATT5TA OF THE [May 1909, 



locality and horizon, the gritty ash (322) a third of a mile south-south- 

 west of Shangort. The affiuity of this species with Streptis moniUfera 

 (M'Coy), 1 which occurs in the Upper Bala Limestone of the Chair 

 of Kildare, is obvious ; but it differs in the greater number of con- 

 centric ridges (25 instead of eight or nine), in the tubercles upon 

 them being widely separated instead of closely placed or contiguous, 

 and in the interspaces between the ridges being narrow impressed 

 lines instead of wide concave grooves. The internal characters of 

 Streptis have not been previously described, and we may note in 

 them some resemblance to those of Mimulus aungloJcensis, Reed," 

 from the Silurian of Burma, especially in the presence of the umbonal 

 chamber ; but there is in this Irish species of Streptis no median 

 septum between the diductors. The only other known species of 

 Streptis in the British Isles is Str. grayi, Dav., from the Wenlock 

 Series, but this is asymmetric, and the concentric ridges are 

 fimbriated. 



EXPLANATION OF PLATES IV-YE. 

 Plate IV. 

 Geological map of the Tourrnakeady District (County Mayo), on the scale of 

 6 inches to the mile. 



[All the figures are magnified 28 diameters,] 



Fig. 1. Quartz-felsite (73). Derrassa. The main part of this figure is occupied 

 by a large strongly-corroded quartz-crystal. Parts of two much- 

 weathered felspars are seen. The little pale spots in the ground-mass 

 are principally quartz. (See p. 133.) 



2. Hornblende-lamprophyre (21). Gortanalderg. The figure shows 



one well-terminated cross-section of a. hornblende-crystal, exhibiting 

 the two cleavages. Parts of another large and several smaller horn- 

 blendes are seen. The rest of the figure consists principally of quartz. 

 (See p. 137.) 



3. Olivine-dolerite (78). West of Gortbunacullin. The most prominent 



objects are the labradorite-crystals. Between these are small 

 grains of augite, olivine, and magnetite, not clearly differentiated in 

 the figure. (See p. 137.) 



4. Spilite (3). Half a mile north of the Lodge, Tourmakeacly. The 



most prominent object in this figure is an amygdule, the main part 

 of which is formed by quartz, while external to this comes a double 

 layer of chlorite, and finally a discontinuous layer consisting partly 

 of calcite, partly of epidote. Surrounding the amygdule are numerous 

 little felspar-crystals, which extinguish practically straight and are 

 probably oligoclase. (See p. 136.) 



5. Calcareous tuff (320). West of Shangort. A quartz-grain and parts 



of several felsite-lapilli are seen embedded in a calcareous matrix. 

 (See p. 138.) 



6. Eadiolarian chert (330). West of Gortanalderg. Sections of seven 



radiolaria are seen wholly or in part. (See p. 111.) 



1 F. M'Coy, 'Silur. Foss. Ireland' 1846, p. 25 & pi. iii, fig. 3 ; T. Davidson, 

 ' Monogr. Brit. Foss. Brach.' vol. iii, p. 200 & pi. xxv, figs. 3-5, Palasont. Soc. 

 vol. xxii (1868). 



2 F. R. C. Reed, ' Palajont. Indica' n. s. vol. ii, Mem. 3 (1906) p. 100 & pi. vi, 

 figs. 13-17 a. 



