500 H. G. Smith— Basic Intrusions 



givea for the belief that this Echinoid is probably a Clypeastroid, 

 and one of the Fibulariidse. In view of its undoubted resemblance 

 to the Holectypoida, particularly to the Pygasteridse and Discoidiidse, 

 Pygastrides is regarded as affording ontogenetic evidence of the 

 phyletic connection of the Clypeastroida and Holectypoida through 

 the Discoidiidae. 



EXPLANATION OP PLATE XVII. 

 Fig. 



1- — External view of the peristomial region of Pygastrides relictus, much 

 magnified (after Loven) . 



2. — Internal view of the same region. The perignathic processes are broken 

 away in areas 16, IVa, and Va. 



3. — Proximal end of an ambulacrum of a Scutellid (Encope), showing the 

 single, perradial, deeply sunken sphasridial pit (after Lov6n). 



4. — Plan of the peristomial region of the corona of Echinocyamus, internal 

 surface. The ambulacra are stippled. (Eeversed and modified from 

 Loven so as to compare with Fig. 2.) 



5. — Adapical view of young Ahatus cavernosus, 2-5 mm. in diameter (after 

 Mortensen). The periproct is Plesiechinoid in position, and the apical 

 system Pygasterid in character. 



6. — Internal view of the peristomial I'egion of young Echinoneus, 4 '19 mm. 

 long (after Westergren). Note the reduced proximal plates and the 

 progressive ambulacral structure. 



7- — Diagrams to suggest the possible origin of the Fibulariid "auricle". 

 The ambulacra are stippled, (a) Disjunct processes of Pygaster or 

 Pygastrides. Eetractor muscles single, protractors on edge of proximal 

 interambulacral plate, with or without a rudimentary ridge. (6) Pro- 

 cesses meeting across interradius, and based upon the interambulacral 

 plate. Eetractor muscles double, protractors on a raised (Holeetypoid) 

 ridge, (c) Auricle of Echmocyamus. The dotted lines indicate its 

 possible origin from stage h. 



IV. — Thk Basic Intrusions East of Gelli Hill, Radnorshire. 



By PI. G. Smith, A.E.C.S., B.Sc, F.G.S. ; with three analyses by 

 J. H. Williams. 



(PLATES XVIII AND XIX.) 



DURING the last few years I have spent a considerable amount of 

 time studying the geology of the country east of Llandrindod, 

 but much still remains to be done, and the present paper merely 

 embodies a few of the points which seem to be satisfactorily 

 established with regard to a small portion of the area. Three 

 distinct types of igneous rocks are recognized, and some facts and 

 ideas with regard to each are put forward. 



The Olivine Basalt. 

 Forming part of the N.N.E.-S.S.W. ridge east of Tyn-y-coed, and 

 best exposed in a quarry at its southern end, is a dark- coloured, 

 almost black, fine-grained igneous rock, which, in the absence of any 

 published descriptions,' calls for some comment. , . The same type 

 also occurs about a mile to the north in the neighbourhood of Bwlch- 



' Dr. Harker (Presidential Address to the Geological Society, Q.J.G.S., 

 pt. i, 1917) mentions the existence of basalts in the Wells country, and 

 considers them to be extrusive. 



