ORIGINAL ARTICLES. 



Morphological Studies on the Eehinoidea Holectypoida 

 and their Allies- 



X. ON APATOPYGUS GEN. NOV. AND THE AFFINITIES OF SOME 

 RECENT NUCLEOLITOIDA AND CASSIDULOIDA. 



By Herbert L. Hawkins, D.Sc, F.G.S., Professor of Geology, 

 University College, Reading. 



(PLATE VII.) 



1. Introduction. 



T17HILE collating the plates in a recently acquired copy of tlie 

 ^' Revision of the Echini (A. Agassiz, 1872-4), I noticed with 

 surprise a representation of complex ambulacral structure in 

 " Echinohrissus " recens (pi. xiv a, figs. 3-4). A style of plate- 

 grouping essentially similar to that in Echinoneiis (tab. c.it.,figs. 7-8) 

 is portrayed as affecting all the extra-petaloid parts of the five 

 areas, and encroaching slightly on the distal j)arts of the petals. 

 -No reference to this unusual condition occurs in the text. As the 

 proofs of my recent paper on ambulacral structures (Phil, Trans., 

 B, vol. 209) were in my hands at the time, I was immediately 

 impressed by the remarkable resemblance between " E." recens 

 and the Cretaceous Tremato'pijgus — a correspondence by no means 

 restricted to ambulacral plating, but involving all coronal features 

 save the orientation of the peristome. 



Within an hour of the discovery a letter was dispatched to Dr. H. L. 

 Clark, of the Museum of Comparative Zoology, Cambridge, Mass., 

 asking if he could confirm the presence of the structure indicated, 

 and begging for any fragments oi" E." recens that could be spared. 

 By return mail he sent me entire tests of three rare Echinoids, 

 " E." recens (to him an Oligopodia), Oligopodia epigonus, and Rhyn- 

 chopygus carihoearum. He encouraged, nay commanded, me to 

 work my will on the beautiful specimen of the first-named species, 

 even if pulverization were the result, and placed but few restrictions 

 on my treatment of the others. Some idea of the disinterested 

 generosity shown can be conveyed on two counts. In the first place 

 the specimens are recent forms on which Dr. Clark has been working 

 lately, thereby " staking a claim " in them whose justice all would 

 admit. But he handed them over to the tender mercies of a palaeon- 

 tologist who had not the temerity to ask for them ! Secondly, 

 Agassiz Museum possesses only four specimens of " E." recens 

 (one of which I was instructed to destroy), two of 0. epigonus (the 

 better of which, figured in the Revision, pi. xix6, figs. 4-6, is in 

 my hands), and two of R. carihoearum. 



The liberty and fraternity of science could scarcely have had more 

 convincing exposition. In publicly expressing my gratitude to 

 Dr. Clark and the Museum with which he is connected, I feel 



