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II. — Structure of Podura Scales. 



By F. H. Wenham, Vice-President E.M.S. 



Mr. MoIntiee has kindly presented me with a number of sHdes ; 

 two of them are superb specimens of the test Podora, L. curvicollis, 

 with markings remarkably dark and distinct. In one of the best 

 scales there is a singular case of fracture, evidently caused by the 

 slipping of the cover, which has dragged the specimen asunder near 

 one-third from the upper end, which is separated to a short dis- 

 tance. Projecting from 

 the largest portion is a 

 fragment, partly torn 

 away, as shown by 

 the accompanying cut, 

 which is an outline 

 sketch taken by the 

 camera lucida, 4000 

 diameters. On the 

 right-hand extremity 

 the longitudinal tear 

 has taken place close 

 to a rib, or marking, 

 which is nearly iso- 

 ^''^^'^ lated. The transverse 



tear at the bottom leaves most of the ends of the ribs exposed and 

 projecting, as the membrane has torn away from behind them. The 

 extremities of ribs 4 and 5 (counted from the right) are j)articularly 

 plain and prominent. I am willing to submit this specimen to the 

 inspection of anyone doubting the fidelity of the tracing. 



How it can be maintained or admitted that these waved or con- 

 stricted ribs which give rise to the "note of exclamation" markings 

 are " illusory," I am at a loss to imagine. The question can only 

 be determined by fragmentary pieces; but in the Podurse, from 

 the toughness and absence of brittleness in the scales, these are very 

 difficult to obtain, and it is seldom that a fortunate accident occurs 

 in the way shown. It was by fragments that the structure of the 

 diatoms became known, wherein the silicious nodules could be 

 traced down to a single atom, and their form is not now a question 

 of dispute. 



]\Ir. Hennah, and others, have demonstrated how a number of 

 difierent patterns, not the least indicative of real structure, may be 

 produced by transparent bodies of regxdar form, such as glass rods 

 and bosses, &c. Much more incomprehensible would these appear- 

 ances be if the transparent ribbings were twisted, lobated, or in the 

 peculiar form of a Podura marking. The patterns would be innu- 

 merable with oblique light, and the whole area might be made to 



