FOSSIL COCKEO ACHES OF NOETH AMEEICA. 151 



were larger, on the average, than the Blattinariœ, a fact due in great part to the younger 

 cockroaches being all Blattinariiie, for the fore wings of the twenty-nine Mylacridœ average 

 27-5 mm., while those of the one hundred and four Blattiiiarise average 22 mm. only. That 

 even this last is greater than the average size of living cockroaches, one familiar with the 

 latter would readily venture to assert ; but to put it to a fair test, I have estimated the 

 average size of recent species from the measurements given in Brunuer von "Wattenwyl's 

 Système des Blattaires (1865), the last general work on the subject. About 380 species are 

 included in this work, but of only 239 are measurements of the length of the wings given, 

 and from these I estimate the average length of the fore wings of living cockroaches to be 

 18-8 mm., which is distinctly less than the size of the PaUeozoic forms. 



This however is by no means the whole of the story. I have further tabulated sepa- 

 rately the length of the fore wings for the difterent American species from the Millstone 

 Grit to the Trias inclusive and find that there is a marked and regular diminution in 

 average size from one period to another, as will appear from the following measurements of 

 the fore wings, given in millimetres. 



Millstone Grit (3 species), 26-38 ; average 31. 



Lower Productive Coal-measures (39 species), 10-61 ; average 29-7. 



Upper Productive Coal-measures (12 species), 163-5-33 ; average 26-4. 



Barren Coal-measures (23 species), 9.75-31.5 ; average 23.4. 



Permian (56 species), 8-25-28-75 ; average 16-9. 



Trias (17 species), 6-3-24 ; average 13. 



The only doubt about the exact accuracy of this statement is that the fauna of the 

 Rhode Island coal basin, consisting of twelve species, is included in the Lower, when it may 

 perhaps belong in the Upper, Productive Coal Measures. The average size of the Rhode 

 Island species is 27-3 mm., and that of the Lower Productive Coal-measures without them 

 is 30-7 mm. ; while if the Rhode Island species were added to the Upper series, it would 

 increase the average of that to 26-8 mm. ; but this would still not disturb the regular 

 succession of averages. The average size of the fifty species of the Productive Coal-meas- 

 ures as a whole is 27-4 mm., or almost precisely that of the Rhode Island species alone. 



Let me not be understood as maintaining that the size of cockroaches has been steadily 

 and continuously diminishing from the eai-liest times to the present, l)ut only for that period 

 of time which is here considered, and also, I may add, for the later Mesozoic rocks; for I 

 have elsewhere shown that the average length of the fore wings of European Mesozoic 

 (mostly Liassic) cockroaches was 12-5 mm., which is slightly less than that of the species of 

 the American Trias. It is well known that the great mass of Mesozoic and especially 

 Liassic insects of all orders were of small size ; but the insects of the Tertiaries did not differ 

 in this respect in any noticeable degree from those now living. 



I have further tabulated the relative length of the fore wings in the different genera 

 of ancient American cockroaches separately, both as a whole and in each of the periods in 

 which they occur. The table gives these measurements in millimetres. (See page 152.) 



This table shows that in general, especially where the species are numerous, the same 

 rule holds remarkably under each genus, the average size decreasing with the lapse of time. 

 The only noticeable exception is in the two divisions of the Productive Coal-measures, where, 

 in the genera Paromylacris, Lithomylacris and Etoblattina, the averages are reversed from 

 what they should be under the rule. The other exception (as in Oryctoblattina and in part 



