articulates by its posterior part to the second plate which has the shape 

 of a transversly elongated oval, and with a deep festoon-shaped thickening 

 on the posterior edge (Fig. 302, C). This plate corresponds and is homolo- 

 gous to the second plate of the bottom of the clamp of Mazocraeidae and 

 fulfills the same function. Immediately posterior to the plates just des- 

 cribed a third one is located (Fig. 302, D) which forms a "lip" of the 

 posterior valve in the shape of a very thin singularly shaped petal with a 

 thickened posterior (exterior) edge. Attention is attracted by the fact that p. 430 

 this thickened edge has a characteristic depression in the middle bisecting, 

 so to speak, the edge of the plate into two parts, which possibly indicates 

 the origin of this unpaired formation at the expense of two earlier existing 

 elements (see page 425 ). 



Two lateral, plank-shaped separatenesses (Bychowsky calls 

 all the sclerites separatenesses or plates, nobis) are located along the 

 edges of both connecting plates in the posterior valve (Fig. 302, G, H) 

 articulating with 2 curved parentheses which lie partially in the anterior 

 valve of the clamp along its edges and partially externally into the posterior 

 valve and form springs sinriilar to the ones of Chimaericolidae and 

 Diclidophoridae, and are undoubtedly homologous to them. At the same 

 time one can suppose that these two planks "parentheses" are also homolo- 

 gous to the anterior unpaired lip of Mazocraeidae; however, this could be 

 erroneous (see page 425 ). Small chitinous bands or strips, the number of 

 which varies from 5 on each side to 8-9, lie symmetrically and parallel 

 to each other between the lateral parentheses in the middle plate in 

 Plectanocotyle (Fig. 302, A). In isolated cases, these strips can partially 

 merge with each other and form more or less large membrane -shaped 

 plates which line the anterior surface of the valve. In Qctoplectanocotyle 

 the clamps are arranged in a manner similar to what has been described^ 

 but they differ by the absence of strips of the anterior valve. On the whole, 

 speaking about clamps in Plectanocotylidae we can conclude that they are 

 arranged more simply than the ones of Mazocraeidae and apparently are 

 phylogenetically closer to the initial ancestral forms. 



As is seen from the description of the clamps of Plectanocotylidae^ 

 they sharply differ from the "discocotylid-type" (see page ^25 ) and conse- 

 quently one cannot in any way agree with Sproston (Sproston, 1946) regarding 

 the systematic status of the group under exann.ination. On the other hand, 

 the unification of Plectanocotyle with Mazocraeidae into one family also 

 does not seem correct to us because the differences in the structure of the 

 clamps in both groups are sufficiently important, and besides that there 

 are other important characters and signs such as the structure of the 

 copulatory organ and especially the presence, in Plectanocotylidae, of 

 special outgrowths of the posterior edge of the disc, bearing chitinous 

 hooks; whereas in Mazocraeidae they lie on the posterior undifferentiated 

 end of the disc. 



515 



