NO. 3 CASSIDULOID ECHINOIDS — KIER 3I 



GENERIC DESCRIPTION 



Large, circular, or with blunt anterior margin, pointed posterior 

 as in Clypeus rostratus Agassiz, low or medium height, adoral sur- 

 face depressed or flat; apical system, central or posterior to center, 

 tetrabasal, posterior ocular plates posteriorly prolonged, extending to 

 periproct, or short, sometimes with catenal plates joining apical sys- 

 tem to periproct; petals long, broad, open, with broad, tapering porif- 

 erous zones, outer pore slitlike, all ambulacral plates double pored ; 

 periproct supramarginal, in groove or flush ; peristome pentagonal, 

 slightly anterior to center; bourrelets well developed, phyllodes 

 double pored, with three series of pore pairs in each half -ambula- 

 crum ; no buccal pores. 



Comparison ivith other genera. — In petal arrangement and phyllode 

 structure, Clypeus is very similar to Pygiiriis (Mepygurus), but is 

 distinguished from it by its supramarginal periproct. It differs from 

 Nucleolites in having wider petals with broader poriferous zones, 

 usually a wider test, and much longer and more crowded phyllodes 

 with three series of pore pairs in each half-ambulacrum instead of 

 the two in Nncleolitcs. There are, however, some species that are 

 intermediate in some of these characters, and it is difiicult to know 

 to which of the two genera to refer them. 



Remarks. — Many authors have indicated that C. plotii is the type 

 species of Clypeus, but all of them considered Klein (1734) as the 

 author of the genus. Since Klein is pre-Linnaeus, their designation 

 of a type species has no validity. 



I have studied one specimen of Clypeus rostratus L. Agassiz from 

 the Naturhistorisches Museum at Basel, Switzerland, in which two 

 complementary plates occur in the apical system (text fig. 8) posterior 

 to genital 2 and anterior to the posterior oculars. Having seen only 

 one specimen, I do not know whether or not these plates are typical 

 in C. rostratus. I have included a figure (text fig. 9) of a phyllode of 

 this specimen, and of a specimen of Clypeus altus M'Coy (text 

 fig. 10) studied in the Iicole des Mines, Paris. 



Range and distribution. — Jurassic (Bajocian to Rauracian) of 

 Europe, Africa. 



DESCRIPTION OF TYPE SPECIES 

 CLYPEUS PLOTII Leske 

 Plate 7, figure 6 

 Clypeus plotii Leske, 1778. Klein's Nat. dispos. ech., p. 93. 



Material. — Sixteen specimens studied in the collections of the U. S. 

 National IMuseum, two in the Museum of Comparative Zoology, 

 Harvard. 



