FRUCTIFICATIONS FROM THE MAZON CREEK FLORA 15 



Myriotheca arnoldi Pfefferkorn sp. nov. 



Holotype: pi. 6, figs. 4-6, 8-9; ISM 15383 a and b (Langford Coll.). 



Synchron- Para type: pi. 6, fig. 1-3, 7; USNM 43714 a and b. 



Stratum typicum: Francis Creek Shale Member above Colchester (No. 2) Coal Mem- 

 ber. Carbondale Formation, Desmoinesian Series, Westphalian D. 



Locus typicus: Near Wilmington, Will County, Illinois. 



Derivatio nominis: The new species is named in honor of Professor Chester A. 

 Arnold in recognition of his contributions to our knowledge of Paleozoic 

 plants . 



Diagnosis. — The frond is pinnately divided, probably in several orders, 

 but only the last one is preserved. The pinnules are up to 12 mm long and 6 mm 

 wide. The shape of the lamina is not recognizable but is somewhat sphenopteroid . 

 The pinnules are covered by sporangia on the abaxial side. The axis shows a 

 weak longitudinal striation and is slightly winged. 



Sporangia are free, round, sessile, and 1.0 to 1.4 mm in diameter. There 

 are about 35 sporangia on one pinnule; owing to the lobation of the pinnules, there 

 is a tendency toward the occurrence of some groupings, made up of about eight 

 sporangia. This grouping does not constitute the formation of sori. 



Spores are radial, trilete, and circular in transverse plane, and were origi- 

 nally spherical. The trilete marking is distinct, the suture is simple, and the 

 rays extend about two-thirds the length of the spore radius, or about 26 nm. The 

 rays are of equal length. The exine is laevigate. The spore wall is 3 to 4 |im 

 thick, and the diameters of the spores (29 measurements) vary from a maximum 

 of 91 M-m to a minimum of 71 nm, with a mean of 79 urn. 



Discussion. — Myriotheca arnoldi has the largest sporangia and pinnules 

 so far known in the genus; in other characteristics it is somewhat similar to M . 

 anqlica , M . monomakhoffi and M. permica . The grouping of the sporangia in 

 M . arnoldi appears to be distinctive, because the midrib of the pinnule is not 

 covered by sporangia (text fig. 6B) . 



It is assumed that the foliage of M. arnoldi was Sphenopteris -like, be- 

 cause the veins are bipinnatifid and not stiff or regular. They bear some resem- 

 blance to _Sphejioj)teris txida^tylites. (Lesquereux, 1880, pi. 55, figs. 9 a-b) . 



As dispersed spores, those of Myriotheca arnoldi would be assigned to 

 Punctati sporite s or perhaps Calamospora . The spores have not been correlated 

 with any named species of Punctati s porite s or Calamospora , but except for thick- 

 er spore coats, the spores are similar to Calamospora sp. 1 (Peppers, 1970) from 

 the Cardiff Coal. Spores reported from sporangia of Myriotheca anqlica (Kidston, 

 1923) are very similar to those from M_. arnoldi. 



Genus CORYNEPTERIS Bailey, 1860 



The fructification genus Corynepteris was early recognized as belonging 

 to the zygopterid ferns . Even though the genus is well defined, the synonymy is 



