BY R. J. TILLVARD. 247 



missing between Mj and S^^. But such a cross-vein may well 

 have been present, though not visible in the fossil. If so, then 

 Ml was already linked up and in line with M'; if not, a very 

 slight bulging of Sj, downwards would give the necessary con- 

 nection. Cui is already linked up with Cu', though, it must be 

 confessed, very irregularly But it is just from such irregular 

 and unstable forms as this that the most beautiful and perfect 

 venational specialisations often spring. 



The condition of JVli and M., in this fossil is very interesting, 

 and leads me to suggest that the wing was a forewing. For 

 there is already a weak median arculus present between Mj and 

 Gui (at the point where Mj leaves Mj). Moreover, M, is pecu- 

 liarly curved, so that, of the three elongated cells marked ofi" by 

 cross-veins between it and Mj, the middle is already the narrow- 

 est. If, therefore, at the time when M' definitely functioned 

 with Ml, a further strengthening were needed, it would only be 

 necessary for the middle cell to close completely up, and we 

 should have the exact formation found in the forewing of 

 Chrysopa. 



The other obvious points in which Mesochrysopa diiTers from 

 Chrysopa are just such as we might expect in an older and more 

 generalised type of wing. The distal veinlets in the costal space 

 are forked, so are the endings of R, Rs, and all its branches. 

 Below Cu', the branches of Rs descend a considerable distance 

 before they reach the wing-border, and are separated by 

 two irregularly-placed rows of cross-veins. These cross- veins 

 must have all disappeared, and the distance between Cu' and the 

 border must have become considerably lessened, before this por- 

 tion of the wing could take on the true Chrysopid facies. As 

 for the distal forks, quite a considerable number of them remain 

 in Chrysopa {df in Plate x.), but they have disappeared in the 

 pterostigmatic region, and from the ends of several of the most 

 proximal and most distal of the branches of Rs, leaving only 

 the middle branches forked. We must note also that, in Meso- 

 chrysopa, Rs is still "ungebrochenen." The weakly zig-zag course 

 of this vein in Chrysopa is a well-known form of specialisation, 

 of frequent occurrence— for instance, in the veins of Agrionid 



