8 II. L. Hawkins — Apical St/stem in the Holectypouhi. 



so that eacli passes under the preceding one ; indications of paired 

 round pores in the end wall some way below the surface-level are 

 seen from time to time ; length about "52 mm., breadth about "48 mm , 

 diameter of aperture about "4 mm. 



Ooecia short low and Avide hoods occurring only near the outer 

 margin of mature zoaria, and so inconspicuous that they can only be 

 distinguished with certainty from slightlj^ projecting zooecial walls 

 by looking horizontally along the zoarial surface. 



Basal lamina smooth, divided by more or less radiating sutures into 

 lines corresponding with the real (but not the apparent) distribution 

 of the zooecia on the upper surface ; the outlines of the individual 

 zocecia are almost always quite obliterated ; the method of growth 

 leaves a very characteristic impress at the edge of the zoarium, by 

 which the nature of quite small fragments may often be recognized ; 

 pores occur irregularly in the lamina and are probably connected with 

 the internal pores above mentioned. 



Not uncommon at Ti-imingham, and owing to its robustness often 

 well ])reserved. By the initial stages and the presence of ooecia it is 

 easily distinguished from the only other Cretaceous Later ojlu&trellaria. 



EXPLANATION OF PLATE I. 



(Figs. 1-4 from photographs by the author, Figs. 5-10 from drawings by 

 Miss G. M. Woodward.) 



FiGr. 1. Discofliistrellaria Trimensis. A specimen, natural size. 

 ,, 2. Ditto. A fragment of another specimen, x 12. 

 ,, 3. Ditto. Ditto. 



,, 4. Lateroflustrellaria rohusta. A specimen, natural size. 

 ,, 5. Ditto. The greater part of a very fiat specimen, showing the parallel 



rows of zooecia continuous past the central (initial) zooecium. x 8. 

 ,, 6. Ditto. The initial zooecium and surrounding zooecium from the 



specimen shown in Fig. 5. x 12. 

 ,, 7. Ditto. Ditto, from another specimen, x 12. 

 ,, 8. Ditto. Ditto. 



,, 9. Ditto. Underside of another specimen. x 6. 

 ,, 10. Ditto. Underside of and cross-section through another specimen, in 



which the individual zooecia are still traceable. x 6. 



IV. — On the Evolution of the Apical System in the Holectypoida. 



By Heebekt L. Hawkins, M.Sc, F.G.S., Lecturer in Geology, University 

 College, Eeading. 



(PLATE II.) 



A. Introduction, 



IN^ two previous papers in this Magazine (Hawkins, 1910 and 1911) 

 I have dealt with some features in the comparative anatomy of 

 the ambulacral and interambnlacral areas of the Holectypoida. The 

 only other structure of the test for a comparative study of which 

 sufficient material is available is the apical system. The present 

 paper deals with this structure. I intend to summarize the results 



