JONES CAMBEIAN ENTOMOSTBACA. 183 



during floods along with the nauddy deposits, and so spread out along 

 a shore. They certainly appear as if broken up ; for none of the 

 fibres appear perfect. 



8. The beds of the Menevian group now come in, at first gritty 

 and of a grey colour, and rather unfossiliferous, then becoming darker 

 in colour and of a finer grain, and at last almost a fine black slate. 

 Most of these beds must have been deposited in deep water, as they 

 are not accompanied by sandstones until the very last, near their 

 junction with the Lingula-flags. In the finer beds fossils are 

 very plentiful, and there is also a corresponding increase in the 

 variety of forms. 



9. The true Lingula-flags, a series of sandstones and shales, occa- 

 sionally ripple-marked. Very barren for the most part, and showing 

 a return to shore-deposits. 



These successive changes, producing such varied conditions of de- 

 posit, must have had much to do with causing barrenness in parts of 

 the strata, and with the appearance, on the other hand, of successive 

 zones of animal life. The continuation of the same genera through 

 a great thickness of shore or shallow-water deposits, as is the case in 

 the Longmynd and Lingula-flags, and the rapid dying out and 

 shorter range of the genera in finer beds or deep-sea deposits, like 

 the bulk of the Menevian group, are interesting facts, and deserving 

 cf consideration when we seek for natural laws to account for the 

 conditions presented to us at these early periods. 



Note on the Entomostkaca /row the Cambrian Rocks of St. David's. 

 By Prof. T. Rupekt Jones, F.G.S. 



1. Lepeeditia Hicksii, Jones. PI. Y. fig. 16 (reversed and imperfect). 

 Small, ovate ; hinge-line short ; posterior margin broadly curved, 



anteriorly rather narrower. Ocular spot indicated by a subovate, 

 smooth, and faintly depressed area, reaching up to the hinge-line. 

 Muscle- spot marked (on one side) by a faint irregular subcentral 

 depression, whence a branching vein-like line of bright pyrites runs 

 downwards nearly to the ventral border. Surface mostly punctate 

 by the exposed cellular structure of the test, which has been con- 

 verted into iron-pyrites, and nearly bared of its outermost layer. 



This is the smallest Leperdltia I know, and differs from others 

 chiefly in its very nearly oval outline, and in the relative largeness 

 of its eye-spot. 



The still smaller Pr^m^7^a solvensis was at first classed as a Leper- 

 ditia, but has been subsequently referred to a more appropriate genus. 



The above-described old Cambrian Leperditia is here named after 

 its discoverer. 



2. Entomis btjpPvEstis. pi. V. fig. 15. 



Leperdltia bupestris, Salter, Brit. Assoc. Hep. 1865, Trans. Sect. 

 p. 285. 



L. punctatissima, Salter, Siluria, 1865, Appendix, p. 519. 



This is nearest to EHtomis divlsa, Jones (Monthly Microsc. 



2 



