BEYRICHIIDAE 



35 



However, because of the simple hingement 

 and the ovate outline with arched dorsum, 

 it is believed to be most closely allied to 

 the Bairdiidae. 



Tetratylus ellipticus Cooper, n. sp. 

 Plate 5, figures 1-6 



Carapace w^ith symmetrical lateral out- 

 line, elliptical, greatest height and length 

 central; dorsal outline somewhat wedge- 

 shaped ; greatest thickness near posterior 

 end ; terminal ridges fairly prominent ; 

 spines short knob-like ; sulcus shallow just 

 back of middle ; surface faintly punctate. 



Length, 0.58 mm; height, 0.33 mm; 

 thickness, 0.23 mm. 



Paint Creek formation, locality 24, 

 abundant. 



This species may be distinguished from 

 T. menardensis (Croneis and Bristol) by 

 its bilaterally symmetrical outline, and 

 from T. elongatus n. sp. by the lower form 

 ratio (length over height). 



Tetratylus elongatus Cooper, n. sp. 



Plate 5, figure 7 



Carapace elongate, ends rounded and 

 equal ; terminal ridges absent or very faint ; 

 spines short, knob-shaped; sulcus shallow, 

 almost central ; surface punctate. 



Length, 0.58 mm; height, 0.28 mm. 



Paint Creek formation, locality 24, rare. 



Tetratylus menardensis 



(Croneis and Bristol) 



Plate 5, figures 8-12 



Healdia? menardensis Croneis and Bristol, 1939, 

 Bull. Denison Univ., Jour. Sci. Lab., vol. 34, 

 p. 98, pi. 3, fig. 28, Menard formation. 



Length, 0.52 mm ; height, 0.28 mm ; 

 thickness, 0.22 mm. 



Paint Creek formation, locality 24, 

 common ; Menard formation, locality 28, 

 depth 748 feet, common. 



There are apparently three fairly distinct 

 Tetratylus species in the Illinois Chester : 

 ( 1 ) The bilaterally symmetrical form, T. 

 ellipticus; (2) the elongate T. elongatus, 

 and (3) a short, asymmetrical form, T. 

 menardensis. The latter differs from species 

 of Healdia in possessing a sinus, four nodes, 

 and a reversal of overlap. 



Family Beyrichiidae Ulrich 



Genus Beyrichia McCoy 



Beyrichia contracta Cooper, n. sp. 



Plate 5, figures 13-14 



Carapace elongate, thin anteriorly in 

 lateral view; dorsum thin, expanding to 

 wide venter. Three nodes near dorsum, the 

 middle one wider, but not so high as an- 

 terior one; fold confined to border of ven- 

 tral margin, separated from posterior node 

 by a low saddle. 



Length, 0.65 mm; height, 0.37 mm; 

 thickness, 0.35 mm. 



Golconda (basal Okaw) formation, lo- 

 cality 18, rare. 



Differs from B. placida Croneis and 

 Gale in the very narrow anterior end in the 

 lateral view. 



Beyrichia sagitta Cooper, n. sp. 

 Plate 5, figures 15-17 



Carapace subrectangular, with distinct 

 backward swing; free margins bordered by 

 frilled false margin which is usually more 

 or less completely broken away; three 

 nodes, two located on either side of the 

 center near the dorsum, the third at anterior 

 end of dorsal fold which rises rapidly from 

 posterior end, giving dorsal outline the 

 shape of an arrow point ; dorsum thin, keel- 

 like; venter wide. 



Length, 0.55 mm; height. 0.28 mm; 

 thickness, 0.21 mm. 



Paint Creek formation, locality 2, rare. 



This species differs from B. placida 

 Croneis and Gale in the extreme angularity 

 of the dorsal outline. 



Family Cytherellidae Sars 



Genus Cavellina Coryell 



Cavellina bransoni (Morey) " 



Plate 5, figures 22-23 



CytJierella bransoni Morey, 1935, Jour. Paleon- 

 tology, vol. 9, p. 482, pi. 54, fig. 5, Amsden 

 formation. 



Length, 0.75 mm; height, 0.50 mm; 

 thickness, 0.31 mm. 



Kinkaid formation, locality 3, common. 



Following the ideas of Geis (1932) and 

 Kellett ( 1935), this species is classified with 

 the Carboniferous Cavellina rather than 

 with the Mesozoic Cytherella. 



