366 



HOLECTYPUS. 



are small pentagonal bodies, wedged between the angles formed by the ovarials and the 

 summits of the ambulacra ; the eye-holes are large, and pierced near the lower border 

 (fig. 2 e), their axis having a slanting direction upwards. The surface of all the 

 discal elements is covered with close-set, miliary granules, similar to those on the surface 

 of the plates. 



The vent occupies the margin of the single inter-ambulacrum (fig. 2 c) ; its form is 

 pyriform, with the rounded base extending halfway into the ventral surface (fig. 2 h), and its 

 apex directed upwards towards the apical disc (fig. 2 c, g). The widest or basal portion 

 of the vent occupies the outer half of the space between the peristome and the margin 

 (fig. 2 b), and the apex extends one fourth of the distance between the base and the 

 disc (fig. 2 ff), but these proportions vary considerably in every individual I have 

 examined ; the marginal opening for the vent, however, is a constant diagnostic character 

 of the species. 



The mouth opening is small, being rather more than one fourth the diameter of 

 the test ; the peristome is equally decagonal, the notches are wide and rounded, with the 

 margin reflected (fig. 2 b) ; the base is concave ; and the oral opening lies in a central 

 depression of the ventral surface. 



Among the many hundreds of specimens which I have examined, the specific 

 characters above described are retained in all with remarkable persistence ; still there are 

 certain points, such as differences in the amount of convexity, the proportion between the 

 height and the breadth, the distance between the vent and the mouth, and the number 

 and regularity of the tubercles, which vary in certain specimens ; they may all, however, 

 be reduced to three principal types. 



Var. a. HemisphcBricus, represented by our fig. 2 «, b, c. 



Var. j3. Conicus, represented by our fig. 2 ff. 



Var. y. Depressus, with the upper surface much depressed, but the vent marginal. 



In the following table I have embodied the comparative dimensions of six specimens 

 from six different localities in Dorsetshire : 



No. 



Locality. 



Height. 



Breadth. 



Anus to Mouth. 



Inch. 



■ Inch. 



Inch. 



1 



Stoke Knaps ..... 



A 



1 



i^ 



2 



Loders ..... 



tV 



i-h 



A 



3 



Crewkerne ..... 



^ 



ItV 



T" 



4 



Castle Gary ..... 



^ 



A 



A 



5 



Bridport Harbour .... 



t\ 



■h 



A 



6 



Walditch ..... 



Q 



T2 



ll^ 



-h 



