4 Herbert L. Hawkins — Studies on the Echinoidea. 



forwards into a thin subtubular peristome. There is a tendency for 

 a roof to arise along the interzocecial furrows and spread over the 

 posterior ends of the zocecia. 



Ocecia very scarce, large, and very globose ; aperture narrow, 

 vertical. 



Avicularia. — {a) Vicarious, very large, up to 1 mm. in length, of 

 hour-glass type, with very round anterior ends and side walls sloping 

 outwards in the middle; apertures rounded posteriorly and almost 

 pointed anteriorly, with a large area of internal front wall in the 

 anterior bulb of the hour-glass and a considerable amount in the 

 posterior bulb; there is athick transverse bar just below the area of 

 constriction, {h) Accessory, small beak-shaped or rounded masses 

 raised above the general surface apparently on slender legs as in 

 M. Sherhorni, Bryd. {ante), and scattered in great abundance along 

 the interzocecial furrows, while sessile specimens encroach very freely 

 on the broad posterior lip of the aperture. 



This very fine species occurs sparingly in the Trimingham Chalk. 



EXPLANATION OF PLATE I. 



(All figures x 12 diams.) 



Fig. 1. — Memhraniporella AUonensis. Zone oi Holaster ijlanus, nesir Alton, 



Hants. 



2. ,, ,, Another part of the same specimen 



showing closed avicularia. 



3. ,, Bedliaviptonensis. Zone of B. mucronata, Bed- 



hampton, Hants. From the 

 margin of a large zoarium. 



4. ,, ,, Part of the interior of the same 



zoarium. 



5. ,, ,, Part of a zoarium in which no 



avicularia have developed. 



6,7. ,, Shawfordensis. Zone ot A. qziadratus, Sh&wiord, 



Hants. 



8-10. ,, Trhnensis. Zone of B. mucronata, Trimingham. 



Different parts of the same 

 zoarium. No. 10 shows an 

 ocBcium slightly confused 

 with a perfect avicularium. 



II. MoilPHOLOGICAL StUDIKS ON THE EcHINOrDEA HoLECTTPOIDA AND 



THEIR Allies. 



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

 College, Reading. 



YII. The Perignathic Girdle of the Holectypoida and the 

 Corresponding Structures of other Irregular Echinoidea. 



(PLATE H.) 



1. Introduction. 



11^ the three preceding papers of this series descriptions have been 

 given of the perignathic girdles of familiar representatives of the 

 three families into which the Holectypoida are at present divided. 



