THE 



G K O LO GI C A L M AG A Z I N E 



NEW SERIES. DECADE VI. VOL. I. 



No. L— JANUARY, 1914. 



I. — Some Pkoblematical Structukks in thk Holectypoida. 



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

 University College, Reading. 



(PLATE I.) 



D TIRING a detailed examination of the tests, and in particular of 

 the ambulaci'a, of the common British species of the Echiuoidea 

 Holectypoida, I have found two series of structures the correlation 

 of which, with those known in recent forms, seems difficult. Since 

 I am engaged in the preparation of a description of the British 

 Pygasteridse, I have thought it advisable to publish now an account 

 of these peculiar features, in the hope that others may be able to 

 throw light on points that seem to me obscure. 



The structures to which attention is drawn are both of the nature 

 of sculptured pits on the coronal plates, and seem to be restricted to 

 the adoral surface. One series of pits is arabulacral in position, the 

 other interambulacral. The former series is not associated with any 

 neighbouring peculiarities ; the latter is so situated that it seems to 

 be in some way connected with the external branchiae. The former 

 set of pits is veiy minute, and is, I believe, recorded for the first time ; 

 the latter is often quite conspicuous, and has been figured frequently, 

 usually without comment, by previous observers. I have found no 

 traces of either series of pits in Conulus, but both are present in 

 Pygaster ' semisulcatus\ P. '■ umbrella\ Holectypus depresms, H. hemi- 

 sphcericus, Coenholectypus serialis, and Discoidea cylindrica. The 

 succeeding remarks are based solely on an examination of those 

 species. 



I. The Ambulackal Pits. 



These I noticed first in P. ' semisulcatus ', but found subsequently 

 to be more conspicuous in S. depressus, and most strongly marked in 

 C. serialis. 



The ambulacral plates of the Holectypoida are always potentially 

 triads, on one member of which stands a prominent tubercle belonging 

 to the main (ambulacral) series. When compression of the plates 

 occurs, the one with the large tubercle generally remains of 

 considerable size, the plate immediately above it becomes a demi- 

 plate, while that below it, while often retaining its primary 

 character, loses in height and may become cuneiform (see PI. I, 

 Fig. 2). In Discoidea and Conulus this plate becomes completely 

 severed from the perradial suture (see PI. I, Fig. 3). In Pygaster 

 the pore-pair of this lowest plate of the triad is generally further 

 removed from the adradial suture than is the case with the other two. 

 Whenever the pits under consideration can b«'^|^By!^tliey:' are^Gund to 



DECADE VI. — VOL. I. — NO. I. /^ ^ 



