322 Arthur Holmes — Petrology of North- Western Angola. 



pp. 115-18, pis. xiv-xvi), it is natural to suppose that the ciliary 

 and podial method was that chiefly practised by those earlier Asterozoa 

 in which the mouth-frame was still but slightly differentiated. 



We have now reached this stage of our discussion : we have seen, 

 on the one hand, that by the middle of the Ordovician Epoch certain 

 Pelmatozoa had acquired a structure of the rays and of the peristome 

 strangely like that of the Asteroidea ; and, on the other hand, that 

 the Asteroidea and, to a less extent, the Ophiuroidea, as they are 

 traced back through the geological periods, approach more and more 

 in the same respect to the structure of the Edrioasteridae. The 

 conclusion that suggests itself is obvious, but not necessarily correct. 

 Many difficulties have to be smoothed away before a genetic filiation 

 between the two groups can be maintained. 



The statement, for instance, that Asteroidea occur in the Cambrian 

 is frequently made, and is always quoted by those who regard the 

 Starfish as a very primitive group, or who deny the possibility of 

 their descent from a form similar to any Edrioasteroid. So long as 

 the term Cambrian was extended to the summit of the Caradocian, 

 the statement was admissible. Nowadays it is misleading. No 

 Asterozoa are yet known below the Middle Ordovician ; Edrioasteroidea 

 are known from the Middle Cambrian, and typical Edrioasteridse were 

 contemporaries of the earliest known Starfish. 



Other difficulties are presented by differences of structure and by 

 certain features of the development. These will form the subject of 

 the next Studv. 



VII. — A Contribution to the Petrology of North- Western Angola. 

 By Arthur Holmes, B.Sc, A.B.C.S., F.G.S., F.E.G.S. 



{Continued from the June Number, p. 272}) 



5. Post-Cretaceous Alkaline Pocks between Senza do Itombe 

 and bango, loanda. 



THE rocks described below were collected by Colonel Andrade, 

 in 1904, on the road which begins at Senza do Itombe on the 

 Loanda-Malanji Pailway, and runs in a north-easterly direction to 

 Bango. The localities, which lie strictly on the road itself, are deter- 

 mined by the following kilometre marks, measured from Senza : — 



km. 

 Nepheline syenite (foyaite) ..... near 24 

 Nepheline phonolite (brown) . . . . , , 22 



Olivine ulrichite (camptonitic tinguaite) . . ,,20 

 Nepheline monchiquite (olivine-free) . . . ,, 20 

 Nepheline phonolite (grey) . . . between 16 and 17 



The intrusive rocks penetrate the Cretaceous formations of the 

 district, which gradually die out towards Bango, where the Archaean 



1 In the Explanation of Plate XI, p. 272, the following corrections should be 

 made: Fig. 3, for "wisp-like ferns" read "wisp-like forms"; Fig. 4, for 

 Phenolite ' ' read ' ' Phonolite " ; f or " magnification 40 in each case ' ' read 

 magnification as indicated in each case ' ' . 



