364 New Brad iiopod from the Kuckers Stage in Estonia. 



Well-developed hinge-plates enclose deep sockets for the teeth of the 

 ventral valve, and these plates proceed anteriorly, forming elevated 

 platforms for the adductors, well-defined for the anterior pairs, 

 the first slightly folded. The muscular area is divided by an elevated 

 septum which is highest in the middle of the length of the shell and 

 ends at the beginning of the elliptical visceral areas, which are 

 slightly concave, and show fine striations, strongest in their middle. 

 From the visceral fields the valve bends more or less abruptly towards 

 the anterior and lateral margins. 



External Ornament. — The stronger striae or ribs appear to be hollow 

 and when slightly weatl-ered on the surface give the appearance 

 of double striae. Between any two of these there are four to six 

 rows of longitudmal elongated pits or punctures. Anteriorly the 

 shells have several irregular longitudinal undulations. 



Leptestia sp. 



Material. — One ventral valve (interior), Geol. Mus. Univ. Tartu. 



Locality and Horizon. — Eakvere (Vesenberg), Rakvere stage, E. 



Measurements. — Greatest width . . 20 mm. 



Width at hinge-line . 18 mm. 

 Length . . .15 mm. 



Description. — The ventral valve is less convex than that of L. 

 musculosa. The area moderately high, slightly concave ; the narrow 

 delthyrium covered by a deltidial plate. Teeth rather strong, 

 supported by lamellae, which are less strong than those of L. 

 musculosa. The septal process is only slightly marked. The shape 

 of the clearly bounded visceral fields with parallel ribs is rounded- 

 angular, and recalls the shape of those fields in Davidsonia 

 verneuiliana. Exteriorly the ribbing is essentially the same as that 

 of L. musculosa, but there are only two to four rows of pits between 

 any two of the stronger ribs. 



Generic relations. — It is of interest to note the relation of LejMstia 

 in some characters to different members of the Strophomenidae. 

 Rafinesquina alternata ^ (Conrad) shows a very similar platform of 

 the adductor muscles, but the new form differs from it considerably 

 in its cardinal process, visceral fields, and ventral valves. The 

 visceral field of Leptella ^ shows resemblance to that of L. musculosa, 

 but it differs considerably near the hinge-line. Undoubtedly related 

 to Leptestia is Plectamhonites leptelloides Bekker,^ with a very similar 

 visceral field of the dorsal valve, only the latter has no highly 

 developed marks of the adductors and its cardinal process shows 



1 Hall & Clarke, Paleontology of New York, vol. viii, 1892, pi. viii, fig. 9. 



2 Hall & Clarke, op. cit., p. 293, pi. xva, fig. 15. 



^ I am indebted to Professor 0. T. Jones for calling my attention to the 

 fact that P. schmidti var. leptelloides Beklter, described in my memoir 

 (H. Eekker, " The Kuckers Stage of the Ordovician Rocks of N.E. Estonia " : 

 Acta et Comm. Univ. Dorpatensis, A ii, 1, p. 68, pi. iii, fig. 16 ; pi. iv, fig. 12 ; 

 pi. V, figs. 9-13), is a distinct species and I replace the latter name by that of 

 Plectambonites leptelloides Bekker. 



