284 PYGASTER. 



tected this primary error ; so true is it, that unless we compare typical specimens with 

 each other, the highest authorities may be misled by imperfect figures. 



In the ' Echinodermes Foss. de la Suisse,' M. Agassiz* figured as Pygaster umbrella, an 

 urchin found by M. Gressly in "le Portlandien du Jura Soleurois." This figure was 

 reproduced by M. Desor,t in his ' Monographic des Galerites.' In the ' Catalogue 

 raisonne,' however, these authors have changed the name of the Swiss urchin to P. 

 dilatatus, and have given that of P. umbrella to a species from the Coral Rag of Chatel- 

 Censoir (Yonne). 



In the " Etudes sur les Echinides Foss. du departement de l'Yonne," M. Cotteau 

 has given for the first time good figures with details, and a description of this species ; 

 and he has likewise kindly sent me type specimens thereof, so that I have ample materials 

 for comparing our specimens with P. umbrella, Agassiz (1847). Unfortunately M. Cotteau's 

 figured specimen is partly denuded of its shell, and my specimen is only an interior 

 mould, but the general form of the test, and the figure of the vent opening, are well 

 preserved. 



The test of Pygaster umbrella is thick ; in some specimens it has nearly a circular, in 

 others a sub-pentagonal, form ; in some the upper surface is moderately convex (fig. 2 c), 

 in others it is more or less depressed ; in fact, its general outline varies much with age, 

 sex, and external conditions, so that it is difficult to give a general description of its form 

 that will be true for three or four specimens from different localities. 



The test (fig. 2 a, c) was kindly given me by my friend Mr. J. G. Lowe ; it exhibits the 

 var. o, with a circular outline, and is selected in consequence of the fine preservation of 

 the shell on its upper surface. The ambulacral areas are narrow and lanceolate 

 (fig. 2 a), the single area and the anterior pair are straight, and the posterior pair are 

 only slightly curved inwards at their upper part. They have two marginal rows of 

 tubercles, which extend uninterruptedly from the peristome to the disc, there are thirty 

 tubercles on each row between the margin and the apex, and from eighteen to twenty at 

 the base ; midway between the peristome and the margin two inner rows of tubercles 

 commence, which extend only one third the distance up the sides, and at the widest part 

 of the areas a few additional tubercles forming a fifth row exist (fig. 2 d). As the 

 marginal tubercles are developed on every third plate, it follows that there are 48 X 3 = 144 

 plates in each area. The poriferous zones are narrow (fig. 2 d), one pair of pores corre- 

 sponding to one ambulacral plate • the septa are narrow, and slightly elevated on the 

 surface ; and there are from six to seven pairs of pores opposite each inter-ambulacral 

 plate (fig. 2 d). 



* Pygaster umbrella. Agassiz, 'Description des Echinodermes Fossiles de la Suisse,' 1" partie, p. 8, 

 tab. 12, figs. 4—6, 1S39. 



Pygaster umbrella. Agassiz, ' Catalogus Systematica Ectyporum Echinodermatum Fossilium,' 

 p. 7, 1840. 



f Pygaster umbrella. Desor, ' Monographie des Galerites,' p. 77, tab. 12, figs. 4 — 6, 1842. 



