558 Forty-fifth Report on the State Museum. 



divided by a low median ridge, which bifurcates at its extremity, 

 leaving between its branches a small central muscular impression. 

 This impression is bounded on its sides by a crescentic muscular 

 fulcrum, which extends parallel with the margins to the anterior 

 portion of the shell. At a point back of their centers each gives 

 off a transverse branch extending inward and backward. The 

 portion of the valve lying between the anterior horns of the 

 lateral semilunes is also deeply depressed, but its surface shows 

 no markings. In the brachial valve the cardinal area is also well 

 developed and distinctly grooved. The limits of the muscular 

 scars are indicated by three septal ridges, one axial and one at either 

 side. All these ridges are bifurcated at their anterior extremities. 

 Type, Zeptobolus lepis Hall. Upper Silurian. Three species 

 have been described, all from the Hudson Kiver group and Utica 

 slate of North America. 



Paterula, Barrande. 1879. 

 (Plate 3, Fig. 15.) 



Shells small, oval, with broad margins of contact. Pedicle 

 notch posterior and marginal. Interior of the pedicle-valve with 

 two or four short, faint ridges diverging from the beak. Exter- 

 nal surface concentrically striated. 



Type, Paterula Bohemica Barrande. Lower Silurian (Etage D). 

 These shells are but imperfectly understood. Besides the single 

 Bohemian species a somewhat similar form occurs in the Llandeilo 

 of Great Britain, and in a lower Silurian limestone in the city of 

 Quebec, Canada. 



Obolus, Eichwald. 1829. 



Synonyms ; Vngula, Pander, 1830 ; Aulonotreta (j)arti??i), 

 Kutorga, 1847; Ungulites, Bronn, 1848; Quenstedt, 1871. 



Shells externally much like those of Oeolella, but generally 



larger and somewhat flatter. 

 The valves are unarticulated, 

 both having broad, grooved 

 cardinal areas, though the 

 groove on the brachial valve 

 is sometimes obsolescent. The 

 tt •«, p oboiuBAptxiM*. interior of the pedicle-valve 



Fio. S.M7 — Pedicle-valve. After Davidson. % ' 



In: ~MK -Brachial valve. After Kutorga. sllOWS a pail' of Well-defined 



cardinal or posterior adductor impressions, just behind the 



L10 



