Herbert L. Hawkins — Studies on the Echinoidea. 389 



just above them. In most cases the two transverse rows of scales 

 diverge at their lower end to admit the insertion of this fin and the 

 intercalation of one scale behind it. One ventral ridge-scale at this 

 point thus bears two facettes, one for its corresponding normal flank- 

 scale, one for the intercalated scale. In Pycnodus and the so-called 

 Palceobalistum ponsorti, one transverse row of scales seems to bifurcate 

 at its lower end to accommodate the pelvic fin, and there is a gap in 

 the ventral ridge-series immediately beneath it. The position of the 

 anus is uncertain. 



It is interesting to add that both in Mesodon and Microdon one or 

 perhaps two small rhombic ganoid scales remain on the upper 

 caudal lobe. 



As to the systematic arrangement of the Pycnodonts, it is clear that 

 the earliest species usually referred to Mesodon are the most primitive. 

 I have placed them in a distinct genus in the forthcoming part of my 

 Monograph of Wealden and Purbeck Fishes. Gyrodus and Mesturus 

 are closely related to each other and diverge in several respects from 

 other members of the group. The well-armoured Cretaceous forms, 

 Coccodus and Xenopholis, are also peculiar. The general trend of 

 specialization in the family seems to be as already stated in the 

 British Museum Catalogue of Fossil Fishes. 



EXPLANATION OF PLATE XXIV. 



Pycnodus platessus, Agassiz ; photograph of nearly complete fish in lime- 

 stone, showing a mass of comminuted shells within the abdominal 

 region. Nat. size. Upper Eocene : Monte Bolca, near Verona, Italy. 

 British Museum, No. P. 1633. 



II. — Morphological Studies on the Echinoidea Holecttpoida and 



their Allies. 



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



College, Reading. 

 V. The Perignathic Girdle of Discoides ctlindeicus (Lamarck). 



(PLATE XXV.) 



1 . Introduction. 



PM. DUNCAN, to whose work we owe the standardization of 

 . our knowledge concerning the ambulacral structures of the 

 Echinoidea, published in 1885 x a memoir " On the Perignathic Girdle 

 of the Echinoidea", in which he definitely established the principles 

 whereby this apparatus may be described and put to taxonomic use. 

 The actual term "perignathic girdle", first introduced in that 

 paper, was given with direct reference to the " perfect girdle around 

 the jaws in Discoidea " (I.e., p. 207), although no detailed account 

 of the peristomial structures of that genus was included. In the 

 following year the same author, in collaboration with W. P. Sladen, 

 gave a full desci'iption of the girdle of Discoides, in the light of the 

 results reached in the previous work. The same two authors 

 practically repeated their opinions on this topic in a later paper 

 published in 1889. 



1 See list of literature at the end of this paper. 



