(i) 16 PoLroidolofjia Sinica Ser. B 



Sliell small sul)-so,iiieircular, widT tlmn high, with the podicle valve strongly, 

 and the brachial valvo more gently convex. Hinge-line forming the greatest width of 

 the tfliell, cardinal angles rectangular, sides and front regularly rounded. 



Pedicle valve SDmewhat shar])ly arcuate in transverse section, th<' greatest 

 elevation Ijetween one third and one half tlie distance forward from the beak, which is 

 elevated ami very slightly incurved over tlie art'a. Cardinal area high, arched, and of 

 triangular outline, with a median triangular deltliyrium w hich is higher than its Ijasal 

 Midth. 



Brachial valve sub-semicircular, with a slightly salient beak which is not in- 

 curved. Cardinal area narriiA\', aljout one third as high, in the center, as the pedicle ai'ea. 

 Contour depressed-convex, the greatest elevation about one third the length forward 

 from tJie beak. A very faint median de])ression towards the front. 



Surface of shell marked by strong regular rounded plications, with deep inter- 

 spaces aliout ecpial in width to the plications in the earlier part, but wider towards the 

 front. There are from 19 to 21 of them on the ])cdicle valve, and a corresponding number 

 on the Ijrachial valve. The plications decrease very gradually in size towards the lateral 

 margins, excejrt on the cardinal extremities where the last three or more are fine and 

 narroA\'. Growth-lines \'erv fine and rather obscure. In some s])ecimens the plications 

 become rather widely separated near the' front, as their OM"n width does not increase in 

 proportion. This gives a very marked character to that part of the shell, as compared 

 M'ith specimens in \\hicli the plications thicken more in proportion. 



The dimensions of a nearly complete small speeimen (pi. I figs. 5a - d) are: 

 height of pediele valve 0.2 mm., height of bracliial valve o.T mm., width of shell at 

 hinge area S.5 nnn. Fragments of larger indiviiluals also occur. 



The numerous varieties currently classed under this specific name recjuire a 

 thorough revision, ^\■llen it will probably appear, that there are a numljer of distinct 

 genetic series. The eounnon Upper Ordovician (Caradocian) form of western Europe 

 illustrated l)y Davidson in plate XXXV figs. 1 and 2, is strongly biconvex, and belongs 

 probably to the genus I'Irdorthix which is <lerived from Ortli.h proper liy the increase, 

 among otiier features, of the eonvexity of the brachial valve, until in that respect it is 

 essentially equal to the pedicle valve. On the other hand, in more primitive mutations 

 (primitive at least, so far as convexity of the brachial valve is concerned) the two valves 

 are very unlike, and to this group our specimens Ijelong. That transitional forms connect 

 the two series does not justify us in uniting them, for transitional forms between species 

 of a genetic series are normal and to be expected. The increase in the size of the 



