On Fonail Arthropods in the British Museum. l.')7 



of Siebenrock show in a cunviiicing mnmu r that the eiilar«(e- 

 intMit of tlie plastron in the {^roup C. crueutatuui is due to a 

 secondaiy process. The eiilar-^enieiit not bein;^ attainable 

 by the ^nowtli of the niicKlIe elements, w Iiieh were abeady 

 undeitjoin;^ a reduction in tlie primitive Cinosternida', the 

 terminal elements were called upon in the course of evolution. 

 Tiiis explains why they attain in the Cinosternidie such an 

 e,\cej)tiunal size. But not only the development, but also 

 the articulations, in the plastron of the (.'inoslernidiu differ in 

 re<iaiil to their position from the articulation in other forms. 

 In all tortoises whera tiexibilify of the plastron is developed, 

 us in Sternothivrus, Terrapene^ Ci/clenit/s, and Ptychogaster, 

 the joint is situated on the posterior edge of the hyo[)lastron, 

 while it is on the median suture of the four terminal elements 

 in the Cinosternida?. 



This detail of minor importance is the reason why in all 

 tortoises, except Cinosternidie, only one part of the plastron 

 (either the anterior or the posterior) becomes flexible, while 

 in the Cinosternidto Ijotb parts are movable. 



The single group of tortoises in which the arrangement of 

 the plastral elements might have permitted a double move- 

 ment are those with a large mesophistron {Sternothcerus) , but 

 here, again, the coalescence of the ])elvic girdlo with the 

 posterior plastral element prevents such specialization. 



A curious trait worth mentioning is the fact that in all 

 Cinosternidie the development of the dermal scutes is in no 

 way affected by the change in the underlying bones — so that 

 in this group the dermal elements evidently represent con- 

 servative parts of the body. 



XVII. — Fossil Arthropods in the British Museum. — VIII. 

 Ilomopterafrom Gurnet Bay., Isle of Wiyhl. By T. D. A. 

 COCKERKLL, University of Colorado. 



In 'Annals Entomological Society of America,' 1*J17, p. 13, 

 I estimated that the collections from the Oligocene of Gurnet 

 Bay would yield at least 200 species. At the present montent, 

 if we include the three species described below, the list stands 

 at 154. Perhaps half-a-dozen others have been described 

 and await publication. Having worked over the collections 

 at the British ^luseum, including those sent by ^Ir. Ilooley, 

 I can alfirni that tlie number of species will considerably 



