.140 Reports and Proceedings — Geological Society of London. 



has recently found a specimen of Plychodus decurrens in the zone 

 of Holaster subglobosiis of the Lower Chalk at Glynde (Sussex). 

 Fragmentary remains of both jaws are seen in the specimen, each 

 bearing many of the characteristic teeth arranged in natural order. 

 There are four series, and one small displaced tooth (probably 

 belonging to the fifth series), on the left of the large median series in 

 the lower jaw ; while in the upper jaw the teeth are clearly arranged 

 in six paired series. The specimen proves that the peculiarly 

 effective disposition characteristic of the living Myliobatidee had 

 not been assumed, but that Plychodus more nearly resembled the 

 Trygonidas in its jaws. The probable explanation of the new 

 discovery is, that in the Cretaceous Period the great rays of the 

 ' families ' Myliobatidse and Trygonidse had not become fully 

 differentiated. Professor Jaskel has already arrived at a similar 

 conclusion from general considerations, and has proposed to place 

 all these fishes in one comprehensive family, termed Centrobatidae. 

 If this arrangement be adopted, Ptychodiis represents a primitive 

 sub-family, which still awaits definition from lack of complete 

 specimens ; while the Trygoninse, Myliobatinas, ?ind Ceratopterinas 

 are equivalent sub-families which survive at the present day. 



(2) " On the Igneous Rocks at Spring Cove, near Weston-super- 

 Mare." By William S. Boulton, Esq., B.Sc, A.R.C.S., F.G.S. 



A traverse from end to end of the exposure at the locality shows 

 that the 'basalt-mass' varies in structure and appearance, and that 

 it is by no means a simple lava-flow. It may be roughly divided 

 into three portions. Beginning at the cliff end to the north, the 

 rock for tlie first 30 yards is a pillowy basalt, with tuff and 

 limestone often occupying irregular spaces between the spheroids 

 of amygdaloidal basalt; then, for about 20 yards, the rock is mainly 

 a coarse 'agglomerate,' with lapilli and bombs of basalt and lime- 

 stone ; while the remaining 100 yards or so is an ordinary basalt- 

 coulee, with very few and always small lumps of burnt limestone. 

 The limestone below is reddened and altered, and although tuffy- 

 looking, does not conttiin indubitable lapilli; the limestone above 

 contains lapilli. The pillowy basalt probablj' represents a river 

 of agglomeratic material carrying finer lapilli, larger and plastic 

 masses of scoriaceous basalt, and lumps of limestone, possibly ejected 

 from a vent. The intervening tuff may present an analogy with 

 the 'volcanic sand' of the West Indian eruptions. There is no 

 evidence of the quiet deposition of ashy material, but rather of the 

 tumultuous aggregation of a fluxion-tuff taking place under some 

 depth of sea- water. The large and irregular fragments of limestone, 

 oolitic and fossiliferous, found mainly in the lower part of the 

 basalt-mass, have not come in from above through cracks in the 

 lava, but seem to have been picked up while in a soft and powdery 

 state from the sea-bed in which it had been accumulating, and to 

 have been involved with and altered by the volcanic material. The 

 conditions existing in submarine flows appear to be very like those 

 in a sill or intrusive sheet. 



