130 ME. C. STEWART ON AMBLYPNETTSTES GEISETJS. 



Note on an Abnormal Amllypneustes griseus. By Chaeles 

 Stewaet, F.L.S., Lecturer on Comparative Anatomy, St. 

 Thomas's Hospital. 



[Eead April 15, 1880.] 

 (Plate V.) 

 Heaeing that Prof. P. Jeffrey Bell was about to give before this 

 Society an account of an abnormal Amity pneustes, in which the 

 upper portion of an ambulacrum was deficient, with an associated 

 abnormality of tbe apical system of plates, I thought that a short 

 note on a malformed Atnblypneustes in my own cabinet might be 

 of interest in relation to it. 



The specimen in question is one of the red-spined variety of 

 A. griseus, which measures 16 millims. in height and 19i millims. 

 in breadth. Attention is at once attracted by a crest-like eleva- 

 tion of what appears to be one of the ambulacra, which through- 

 out its entire length is raised above the general level of tbe rest 

 of the corona. At the ambitus it measures 12 millims. in breadth, 

 whilst the width of the four other ambulacra is 6| millims. On 

 examination, it is found that the increased size is really due to 

 its being formed of two ambulacra laying side by side ; each, as 

 usual, is composed of a double row of plates, with an ambulacral 

 area and two poriferous zones. The areas and external poriferous 

 zones are like those of a normal ambulacrum ; but the poriferous 

 zones which touch one another are fused together, with the pores 

 irregularly arranged. The combined poriferous zones are not 

 quite equal to the sum of two normal ones, and they form the 

 most prominent part of the crest alluded to. The apical system 

 is quite normal. Owing to the close crowding of the tubercles 

 and the slight development of the pits at the angles of the plates, 

 the outlines of the latter were difficult to determine. 



The chief interest, then, in this specimen lies in the fact of its 

 abnormality being in direct opposition to that described by Prof. 

 F. Jeffrey Bell (preceding paper), namely, instead of a reduction 

 we have here an increase in the number of ambulacra. The two 

 cases well illustrate Echinoid irregularities, and may lead to other 

 instances being noted. 



DESCRIPTION OF PLATE V. 



Figs. 1, 2, and 3 are referred to in Prof. Bell's paper, antea. 

 Fig. 4. Enlarged diagrammatic side view of corona. A, ambulacra. 



5. Enlarged diagrammatic aboral view. 



6. Portion of abnormal double ambulacrum, X 7 J (sernidiagrammatic). 



