REPORT ON THE ECHINOIDEA. 29 



The type of Holectypus with its regular outline, its buccal cuts, the anal system on 

 the actinal surface, and the diminution of the size of the tubercles to uniformity in the two 

 areas, the restriction of the poriferous zone to a single vertical row, all tend to show that 

 the tendency to the Clypeastroids is already highly specialised. The existence of such 

 forms as Pyrina with their simple ambulacra leads directly to the Nucleolidae and Echino- 

 lampadse. On the contrary, we can only obtain such forms as the present deep sea types 

 from the earlier Cretaceous types like Infulaster, and their derivation from such forms as 

 Colhjrites seems probable, if we take into account such extraordinary forms as have 

 been figured by Ooster (Echin. Alp. Suisses, pi. x. figs. 1-4), as Dysaster calceolatus 

 (see also de Loriol, Echinod. Crdt. de la Suisse, pi. xxxiii.). 



When we take the Spatangoids of the Chalk, they lead us directly through the Palseos- 

 tominse and the CoUyritidae ^ to the Ananchytidse, which have persisted to the present 

 day ; and also to the Spatanginse proper, represented by but few genera, as Mkraster, 

 Hemiaster, and Prenaster, which already possess the structural features characteristic of 

 the recent Spatangoids. That is, we find genera with a peripetalous fasciole, a subanal 

 fasciole, sunken ambulacra, petals of different degree of development, spines specialised 

 on certain areas of the test, a trace of a sunken anterior groove, of an anal beak, of an 

 actinal plastron, of a snout, of a lateral fasciole, and of a specialisation of the primar}^ and 

 secondary tubercles. But, of course, the extent to which these features may be developed 

 in Tertiary and recent genera contrasts often strikingly with the rudimentary nature 

 of the structural features found in the Cretaceous or Tertiary genera. The simple 

 actinostome of the Pateostominse is combined with a well-marked specialisation of the 

 ambulacra above the ambitus, the petaloid feature of the early Spatangoids which appears 

 later than in the Cassidulidse ; while in the Ananchytidse the well-developed labium of all 

 the more recent Spatangidse is combined with a comparatively more rudimentary state 

 of the ambulacral zones. 



Among the Cretaceous genera, Hemipneustes and Ennalaster are extremely instructive. 

 They show, perhaps better than any others, the passage which exists between the earliest 

 Spatangoids with more or less petaloid ambulacra, and the older Spatangoids without 

 petals, and in which the ambulacra have the same simple structure from the apical 

 system to the actinostome. In both these genera the petaloid structure Ls limited to the 

 posterior poriferous zone of the lateral ambulacra ; the only recent genus in which a 

 similar structure still exists is Agassizia. In this genus, however, the posterior lateral 

 petals are normally developed as in other Spatangoids, or perhaps we must consider this 

 as the last trace in normal Spatangoids of the simple condition of the ambulacra, such as 

 we still find it in the Pourtalesise. It is specially interesting to compare these genera first 

 to the Ananchytidfe, then to the Toxasteridse, and finally to such recent genera as Genico- 

 patagus, Homolampas, Argopatagus, and the like. These comparisons lead us to detect 



• The Collyritidse in their turn showing most striking affinities to such genera as Hyboclypus and Galeropygus. 



