Herbert L. Hawkins — Studies on the Echinoidea. 9 



as acute as in Discoides, the general thickness of the test makes 

 unnecessary the pi'olongation of such carinate buttresses. In the 

 Clvpeastroida, where the angle between the adoral and adapical 

 surfaces of the test is often very acute (e.g. Echinarachnius), a 

 bewildering profusion of essentially buttress -like structures is 

 developed. It is worthy of note that in those Clypeastroids which 

 have a moderately rounded ambitus (e.g. JEehinocyamus)t\\Q buttresses 

 are for all intents and purposes retained in the relatively simple 

 " Discoid es-^hase ". 



The buttresses maybe considered to have been developed primarily 

 as supports for the inclined elements of the perignathic girdle, and to 

 have acquired a secondary function as joists, girders, or rafters for 

 the strengthening of the test-fabric. This secondary function is 

 retained, and carried to an almost excessive degree of specialization, 

 in the typical Clypeastroids, where the primary purpose of the 

 buttresses has disappeared, and their original positions are 

 abandoned. 



3. The Cltpeastroid Girdle. 



It is unnecessary to describe in detail the well-known characters 

 of the girdle of the Clypeastroida. Students may be referred to the 

 exquisite di'awings and detailed descriptions in Loven's Echinologica; 

 or, for a general summajy, to Jackson's Phylogeny of the Echini. 

 It will be suiRcient here to indicate the analogies between the various 

 types of Clypeastroid girdle with that of the Holectypoida, and to 

 indicate the probable relations between them. 



The Clypeastroid girdle seems to consist of processes only. These 

 processes are always approximated to one another in pairs near the 

 iuteriadial line, and in many groups are fused into a single, though 

 often visibly compound, element. Their interradial convergence is 

 rendered possible by the reduction in width of the interambulacra 

 as they approach the peristome, and by the actual extension of the 

 ambulacral plates, whereby they sometimes meet internally across 

 the interradius, more or less completely ousting the proximal inter- 

 ambulacral plate from participation in the lining of the test-cavity. 

 Tiie processes may, however, transgress on to the proximal inter- 

 ambulacral plate when this is well represented, this anomaly being 

 usual where the processes are fused. 



In Echinocyamu&, a genus usually regarded as showing arrested 

 evolution, and approximating to the Discoides suhiicuhis group of the 

 Holectypoida, the perignathic girdle is found to be exceptionally 

 specialized. Superficially regarded, it certainly appears like that of 

 D. suluculus, especially since it is buttressed up by the thickening 

 of the interarabulacral plates, and the floor of the test is traversed by 

 carinae. But each section of the girdle consists of a fused pair of 

 processes which are based entirelj- on the interambulacrum. There 

 seems every reason to believe that tlie girdle does actually consist of 

 transposed processes, but there is no proof of the existence of a ridge, 

 whether true or false, included between them. It is certainly 

 questionable whether such a development can be considered primitive, 

 in comparison with that of such a form as Clypeaster. Probably the 



