On an Abnormality in Echinus esculentus. 73 



X. — Note on an interesting Abnormality in Echinus escu- 

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

 iu Geology, University College, Reading. 



During the preparation of a series of young Echini collected 

 at Port Erin, Isle of Man, a small specimen of E. esculentus 

 was found with an abnormal development of considerable 

 interest. Abnormalities often give indications of morpho- 

 logical features that are not apparent in normal cases — ■ 

 a fact that has been shown recently by Jackson (' Phylogeny 

 of the Echini/ Boston, 1912). He gives an analysis of the 

 various types of abnormality commonly found, and the case 

 here described seems to correspond with his "progressive " 

 series. 



The specimen is not quite circular — this disturbance of 

 symmetry being a frequent indication of some more deep- 

 seated irregularity. The peristome is not central, although 

 neither in this case nor in that of the ambital outline is the 

 departure from the normal very noticeable. The diameters 

 of the test, taken from the middles of the ambulacra to the 

 middles of the opposite iuterarnbulacra, are : — 



mm. 



I.-3 24-75 



II.-4 240 



III.-5 24-5 



IV.-l 240 



V.-2 24-25 



The distance of the peristome from the ambitus is 7 5 mm. 

 in area 3 and 6'5 mm. in I. 



From the adapical surface no special irregularity (other 

 than the elliptical outline) can be seen, the apical system 

 being perfectly normal. An adoral view shows that inter- 

 ambulacrum 3 is veiy narrow at the peristome-margin, 

 ambulacra III. and IV. almost meeting across it. A closer 

 examination shows that the interambulacrum is composed of 

 a single plate at the peristomial end. All the other four 

 areas are normally double throughout. 



In all Regular Echinoids whose early postlarval develop- 

 ment is known the interambulacra originate as single plates. 

 This was admirably shown by Loven for Cidaris ( f Echino- 

 logica/ 1892). The single plate is early lost by resorption. 

 It might be thought, therefore, that in interambulacrum 3 

 of this specimen the primitive plate had not been resorbed 

 in the usual way. But this seems not to be the case. A 



