460 MR. HERBERT L. HAWKINS ON 



are proportionately similar to those of Pygaster, but their 

 prominence is almost masked by the extreme elevation of the 

 ridges. In Gomdus this latter feature is carried so far that, 

 except for the suture-line showing that the process is present, 

 the only visible and free portion of the ambulacral part of the 

 girdle is a minute shining knob at each corner of the ridge. 



It is, 1 think, a point of great interest to find that both 

 ingredients of the perignathic girdle are so strongly developed 

 in these later Holectypoid genera. One of the most obvious 

 contrasts between the divisions of the Clypeastroids is the 

 presence in some genera of one support for the jaws in each 

 interradius, and in others of two. It would seem that, when 

 they are double, these supports represent processes, and when 

 single, they are the degenerate relics of ridges. Both such 

 conditions could be obtained readily by the modification of a 

 perignathic girdle in which both portions were equally repre- 

 sented. All that is necessary is a simple process of the elimination 

 of one or the other of the parts. 



Another feature of interest in the pei'ignathic girdle of the 

 Holectypoida, and one connected intimately with the method of 

 use of the jaws, is the angle which the supports make with the 

 plane of the adoral surface of the test. Iq Fygaster the processes 

 are almost, though not quite, at right angles to that surface, 

 with a slight outward slope. This inclination is rather more 

 marked in Anorthopygus, and considerably so in Ilolectyptis. In 

 Discoidea the angle between the girdle and the floor of the test is 

 quite acute, while in Conuhis it becomes, especially in thin-tested 

 forms, almost 45 degrees. The practically vertical girdle of 

 Fygaster would indicate a correspondingly vertical working of the 

 jaws, similar to that of the Regular Echinoids ; while the highly 

 inclined system in Conulus seems to show a tendency towards 

 the horizontal working of the jaws of Clypeastroids. 



In Gonoclypeus, according to the description and drawings 

 given by de Loriol (35), the two ambulacral processes are present, 

 but the ridges have dwindled to insignificant proportions, both in 

 width and height. This brings the processes closely together, 

 and there is consequently induced a markedly Glypeastroid 

 appearance in the perignathic girdle. 



4. The Jaivs. 



Our knowledge of the jaws of the various genera of the 

 Holectypoida is very meagre and unequal. In fact, of the details 

 of the structure of the pyramids and teeth of Jurassic forms next 

 to no evidence is at present available. There are two reasons 

 why this condition of affairs should exist. Firstly, the jaws are 

 internal organs, and so, if they are preserved in the interior of a 

 specimen, it is necessary to break it up before they can be 

 studied. Moreover, it is usual to find the matrix that filtered 

 into the tests of Oolitic forms more compact and refractory in 



