42 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I44 



Comparison with other genera. — Pygurus is most similar to 

 Clypeus, but differs in having an inframarginal periproct. 



Remarks. — There has been considerable controversy over whether 

 or not Mepygiirus was distinctive enough from Pygurus to warrant 

 generic separation, Beurlen (1933, p. 72) and Mortensen (1948, 

 p. 136) considered Mepy gurus as a subgenus of Pygurus. Beurlen 

 suggested that Mepygurus was more primitive than Pygurus, but 

 Mortensen disagreed. The study of the phyllodes (see p. 7) shows 

 Beurlen to be correct. The species shown in chart 3 (p. 8) from 

 the Bajocian to Callovian are species that Lambert and Thiery 

 refer to Mepygurus. They have more primitive phyllodes than the 

 species occurring later that are referred to Pygurus {Echinopygus 

 in Lambert and Thiery). Furthermore, species referred to Mepyg- 

 urus have in general a lower, broader test, a more central apical 

 system, and broader outer poriferous zones in their petals. However, 

 Mepygurus and Pygurus are so similar in their general appearance, 

 and so closely related, that it is advisable to maintain Beurlen's and 

 Mortensen's consideration of Mepygurus as a subgenus of Pygurus. 

 If Mepygurus is considered as a distinct genus, its close relationship 

 to Pygurus might be overlooked. Furthermore, there are some 

 species which are intermediate between the two subgenera. Lambert's 

 (1905b, p. 575) Pygurus perreti has the low, circular shape of a 

 Mepygurus but the broadened phyllodes of a Pygurus, and could not 

 be referred with certainty to either one of the subgenera. 



Evolution. — There is a remarkably consistent trend in the evolu- 

 tion of the phyllodes in Pygurus, with a broadening of the phyllodes, 

 a reduction in the number of pore pairs, and an increase in the 

 distance between the pores and the edge of the peristome. This 

 trend is described and illustrated in detail on page 8, chart 3. 



Besides this evolution in the phyllodes, there are several other 

 trends in Pygurus. The outer pores of the petals are much less 

 elongated transversely in the later species than in the earlier. In the 

 Bathonian specimens of F. depressus the outer pore is greatly elon- 

 gated into a narrow slit (pi. 2, fig. 2), and a similar slitlike outer 

 pore occurs in P. blumenbachi (pi. 3, fig. 4) from the Lusitanian. 

 In the Neocomian species, P. rostratus Agassiz, and P. montmollini 

 (Agassiz) (pi. 2, fig. i) the outer pores are less elongated. This 

 trend reaches its maximum in the Albian and Cenomanian species, 

 P. jagueyanus Cooke, P. lampas (De la Beche) (pi. 3, fig. i), and 

 P. africanus De Loriol, in which the outer pore is much less elongated 

 than in the earlier species. 



