HYATTECHINUS. . 297 



*Hyattechinus beecheri sp. nov. 

 Plate 24, figs. 5-8; Plate 25, fig. 5; Plate 26. 



This very remarkable sea-urchin is known from a single specimen, which is an internal 

 sandstone mold, and bears the impress of the proximal faces of the plates only. The test is 

 depressed, dome-shaped, flattened on the ventral aspect, elongate and bilaterally symmetrical 

 through an ambulacrum and opposite interambulacrum, which I take to be the axis III, 5. 

 The specimen is so perfect in form, without distortion or separation of plates, that it may be 

 taken as representing the original shape. Excepting as shown somewhat in Hyattechinus 

 rarispinus, this is the only regular echinoid known with a central periproct, that presents a 

 bilateral symmetry. The oblong form of the Echinometridae through an oblique axis does 

 not represent a true bilateral symmetry. The specimen measures 26 mm. in height, 80 mm. 

 in length through the axis III, 5, and 71 mm. in width through the peristome and at right 

 angles to the longitudinal axis. While the mold represents only the impression of the interior 

 of the test, it is sufficiently perfect so that I am able to represent the specimen in Plate 26 

 spread out on the Loven method in which each plate was scrutinized with utmost care. At a 

 few points in areas 3 and 4 details could not be made out, and in these areas in part the 

 plates are restored as indicated by dotted lines. The breaks occurring in the figure at the 

 ambitus are due to the mechanical requirements of getting over the edge in a specimen of this 

 shape. Only one ambulacrum is filled in in detail dorsally, and for this the best area was 

 selected. 



The ambulacra are broad and petaloid ventrally, being 13 mm. in width at the widest 

 part, 9 nun. wide at the ambitus, and 6 mm. wide at about the middle of the dorsal side of the 

 test. The plates are relatively wide and high ventrally, narrow and much lower dorsally. The 

 pore-pairs are uniserial, situated at about the middle of each plate. This is a general character 

 of the interior, and on the exterior they would doubtless have been much nearer to the inter- 

 ambulacra on either side (compare Plate 20, figs. 9, 10). The outer pore of each pair ventrally 

 is higher than the inner pore, as in other species of the genus. Between the inner pore and 

 center of the area a slanting gash-like pit exists in each plate ventrally; these pits, being molds, 

 represent elevated nodose spine-like processes which extended inward from the ambulacral plates 

 (Plate 24, fig. 6), similar to those described in H. rarispinus (p. 293), and again, similar to, 

 though less elongate than the spinose processes from the ambulacral plates of Eucidaris and 

 Phyllacanthus (Plate 3, figs. 12, 13). Such spinose processes from ambulacral plates of the 

 corona are known only in the types mentioned, but Sir Wyville Thomson and Mr. Agassiz 

 described similar internal spines as given off from ambulacral plates of the peristome in Poro- 

 cidaris (p. 61). 



The interambulacra are very broad, and make up the greater part of the test, as in all 



