1915- ^*0- 12. THE STROPHOMENIDÂE OF THE tCRISTIAXlA REGION. 75 



Leptaena undata M'Coy (?l. 

 IPI. XIII. fig. 12.I 



1846. Ortliis ufulata M'Coy, Sil. Foss. Ireland, pi. III. fig 21. 



1852. Strophouietta delloidfa, var. rf, imdata, M'Coy Brit- Pal. F"oss., pi. IH, figs. 38 — 39. 

 1871. — »— — — Dadidsox. Brit. Foss. Brach., p. 205. pi. 39. 



fig. 23 - 24. 



A small number of fragmentary ventral and dorsal valves from the 

 Mjesen district may be placed under this term, the specific value of which 

 is not yet sufficiently proved. Of the two figures given by M'Coy in Palaeo- 

 zoic Fossils, one fig. 38 very strongly reminds us of the common Leptaena 

 rhotitboidalis, the other fig. 39 with very faint corrugations only and varying 

 radiating striation. having a more differing exterior. Common to both there 

 appears to be a relatively very small and narrow deflected marginal portion. 



The specimens that 1 refer to Leptaena undata have this character, 

 furthermore a fairly gentle geniculation. a slightly convex central disc, 

 with comparatively faintly developed concentric corrugations. The cardinal 

 angles have been well projected, more acute than shown in M'Coy's 

 figures. The radiating ribs are fairly uniform in the lateral part, whilst in 

 the median portion a number of striae, with varying inter\als. are much 

 stronger than the rest. 



A single specimen in rock from 5 a. Stavnaestangen, Ringerike, has 

 a similar size and outline than those from the Mjesen district, but differs. 

 — besides being still less sharply geniculated — by a very regular striation, 

 consisting of major striae at about equal intervals with a large number 

 (8 — 121 of much finer ones between. The concentric corrugations are very 

 faint and irregular. 



Occurrence. Cyclocrinus horizons of 4, Mjesen district : 5 a. 

 ^Ijesen district land Ringerike ?|. 



Plectamhonites P.\nder 1830 emend. H.^ll and Cl.\rkk 1892. 



In his previously mentioned paper »Die aeltesten silurischen Schichten 

 RuÈlands- La.m.ansky p. 158 — 159 discusses the history of this generic 

 term of Pander's. He reaches the result that the sense in which the term 

 was revived by Hall and Clarke was different from that which Pander 

 had intended. It would preferably have been used for the fossils for which 

 Lam.ansky introduced the generic name Plectella. 



Even if Plectantbonites, however, in the sense of Hall and Clarke 

 does not include eactly the types of shell that P.ander meant by that name, 

 I think the term should be retained in the sense in which it has been 



