(l) 16 Palicontologia Sinica Ser. B 



Shell small sub-semicircular, wider than high, with the pedicle valve strongly, 

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

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



'Pedicle valve somewhat sharply arcuate in transverse section, the greatest 

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

 elevated and very slightly incurved over the area. Cardinal area high, arched, and of 

 triangular outline, with a median triangular delthyrium which is higher than its basal 

 width. 



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

 curved. Cardinal area narrow, about one third as high, in the center, as the pedicle area. 

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

 from the beak. A very faint median depression towards the front. 



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

 spaces about equal in width to the plications in the earlier part, but wider towards the 

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

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

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

 narrow. Growth-lines very fine and rather obscure. In some specimens the plications 

 become rather widely separated near the front, as their own width does not increase in 

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

 with specimens in which the plications thicken more in proportion. 



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

 height of pedicle valve G.2 mm., height of brachial valve 5.7 mm., width of shell at 

 hinge area 8.5 mm. -Fragments of larger individuals also occur. 



The numerous varieties currently classed under this specific name require a 







thorough revision, when it will probably appear, that there are a number of distinct 

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

 illustrated by Davidson in plate XXXV figs. 1 and 2, is strongly biconvex, and belongs 

 probably to the genus Plectorthis which is derived from Ortliis proper by the increase, 

 among other features, of the convexity 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 belong. 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 



