FROM THE UPPER WHITE CHALK. 183 



and oblique (fig. 2/), and the sej)ta between the pores support prominent granuliform 

 elevations (fig. 2g). 



The inter-ambulacral areas are very wide .throughout (fig. 2 c, e, and fig. 3 a, b) ; they 

 are formed of two series of deep plates'; seven in each, that support large prominent 

 tubercles (fig. 2 e, g, fig. 3 a). The four ambital tubercles are the largest; they have 

 well-defined areolas, prominent bosses, and moderate-sized mammelons. A series of mam- 

 millated granules surround the areola, except where it abuts against the zones (fig. 2 

 e and g). The miliary zone is wide, and filled with numerous small granulations (fig. 2 e) 

 in addition to the larger granules that encircle the tubercles. 



The apical disc, two thirds the diameter of the test, has a regular geometrical figure, 

 hence the origin of the specific name (fig. 2 a, b) ; it is circular, slightly convex, and a 

 little elevated at the vent. The ovarial plates have an irregular hexagonal shape, their 

 outer sides are elongated and contracted, and the rounded external border lies within the 

 circle described by the ocular plates, which have an irregular triangular figure, the largest 

 side being turned outwards and slightly undulated (fig. 2 d) ; the sutures are fine, 

 distinct, and regularly interrupted by small punctuated angular impressions ; the oviductal 

 holes open in the middle of the plates, and the orbits lie under the central projecting 

 process (fig. 2 d) ; the surface of all the plates is quite smooth. The vent is slightly 

 elevated, and the periprocte surrounded by a thick annulus (fig. 2 b, d) ; this aperture, 

 nearly circular, occupies the posterior half of the sur-anal and the anterior halves of the two 

 posterior ovarial plates. 



The mouth-opening is one third the diameter of the test (fig. 3 b) ; and the peristome 

 is divided into ten unequal lobes. 



Affinities and Differences. — Salenia geometrica resembles S. scutigera ; by some authors 

 it is considered to be a large variety of the latter. M. Cotteau, in his beautiful Monograph 

 on the ' Echinides du Departement de la Sarthe,' figured and described this Urchin under 

 the name Salenia geometrica, Ag., but in his later and most valuable contribution to the 

 1 Paleontologie Francaise' has united it with Salenia scutigera ; he says, however, 1 as to 

 the Salenia geometrica, " la question est plus delicate et plus difficile a resoudre. Au 

 premier abord, cette espece se distingue certainement du Salenia scutigera par plusieurs 

 caracteres importants : sa taille est beaucoup plus considerable, car sa hauteur depasse 

 souvent 13 millimetres, et son diametre 17 millimetres; sa face superieure est plus 

 elevee et plus sensiblement deprimee au sommet ; ses tubercules inter-ambulacraires sont 

 plus nombreux, et la zone miliare qui les separe plus large, plus droite et plus granu- 

 leuse; ses ambulacres sont plus longs et plus flexueux, et l'appareil apicial, relativement 

 moins grand et moins epais, affecte une forme plus pentagonale. Ces differences se 

 reproduisent chez un certain nombre d'individus avec une Constance qui n'est pas 

 sans leur donner de la valeur ; aussi, dans nos ' Echinides de la Sarthe,' n'avons-nous 

 pas hesite a maintenir la S. geometrica comme une espece parfaitement distincte. 



1 'Paleontologie Francaise, Ter. Cretace,' torn, vii, p. 157. 



