Herbert L. Haiukins — Studies on the Echinoidea. 5 



From the scanty evidence available, it would appear that in respect 

 of this structure Plesiechinus is typical of the Pygasteridse (even 

 including Anorthopygus). The girdle of Solectypus seems essentially 

 similar, thereby differing from that of the Discoidiinse ; but as yet 

 the perignathic structures of the Cretaceous Coenholectypus are 

 unknown. Large species of Discoides have giidles differing in 

 proportions only from those of the smaller forms, typified by 

 B. suhuculns. In default of further knowledge, the development 

 of the girdle in Conulus alhogaleriis may be considered to be that 

 characteristic of the genus and family. It is thus possible to 

 summarize the known features of the girdle of the Order witli some 

 confidence, and to discover the " common denominator" of its varied 

 characters. Stri])ped of its diverse additions, the girdle is found to 

 present a uniform and practically unvaried structure which may be 

 regarded, from the systematic standpoint, as being a diagnostic 

 feature of the Order. 



The primitive and conservative nature of the Holectypoida among 

 Irregular Echinoids has been so often indicated, and indeed is so 

 manifest, that it may be taken for granted without further argument. 

 It follows from this that there are but two alternative possibilities 

 for the lines of descent of all other Orders of the Sub-Class. Either 

 they must have arisen from some "Regular" stock independently 

 o£ the Holectypoida, in which case they need show no special 

 resemblances to that group save under the influence of parallelism of 

 development ; or else they must have descended directly or indirectly 

 from Holectypoid ancestors. In the present paper a brief description 

 of the peristomial characters of certain representative Irregular 

 Echinoids is given, and an attempt is made to indicate the presence 

 or absence of relationship to the Holectypoida shown by this fragment 

 of evidence. The genera whose internal test-structure has been 

 examined for this purpose aie Clypeaster, Echinocyamus, and Echin- 

 arachnius among the Clypeastroida, Nucleolites and Trematopygus 

 among the " Nucleolitoida ", Conulopsis among the ?Cassiduloida, 

 and Eclnnocorys, Micraster, and Echinocardium among the Spatangoida. 

 In the case of the first and last mentioned Orders, the genera studied 

 may be taken as fairly representative of the groups to which they 

 belong; but the other two Orders (usually grouped together under 

 the name here restricted to the latter) are very imperfectly illustrated. 

 This is accounted for by the extreme difficulty of the preparation of 

 the Jurassic forms and the virtual impossibility, under existing 

 circumstances, of acquiring suitable material of the Cassididus— 

 Echinolampas series, which are almost unrepresented in the British 

 fauna, both past and present. 



On strati graphical evidence it is at least possible to regard the 

 Xucleolitoida (including such genera as Nucleolites, Clypeus, and 

 Fygtirus) as "cousins" or even "brothers" of the Holectypoida, 

 rather than as their lineal descendants. If this should prove to be 

 the case an interesting illustration of parallelism would appear. The 

 Holectypoida are first found in the Lias, occur in abundance 

 throughout the succeeding stages of the Mesozoic, and are represented 

 in the Tertiary and Recent faunas only by the apparently degenerate 



