92 LIFE-HISTORIES OF AUSTRALIAN ODONATA, iv., 



gizzard, and very minute. They are arranged radially in 

 eight sets, each set containing from one to six teeth, two 

 being by far the commonest number. The two gizzards 

 shewed remarkable variation, as may be seen by comparing 

 the figures (Figs. 2-3). In No. 1, the arrangement was 

 2, 2, 2, 1, 1, 2, 2, 2. In No. 2, 4, 1, 6, 4, 2, 2, 2, 2. Clearly 

 the number 6 is abnormal. Where more than one tooth ap- 

 peared in a set, the arrangement showed a distinct begin- 

 ning of longitudinal chitinisation ; e.g., in every case except 

 one (the sixth set of No. 2), the second tooth of a set of two 

 is placed under the first, not alongside it. In the two sets of 

 four, found in gizzard No. 2, this tendency is also shewn, 

 for, in one case, the four teeth are arranged longitudinally ; 

 and, in the other, the top pair only are on the same level. 

 In the figures, the teeth are drawn disprojaortionately large, 

 about four times their actual size when enlarged. Their 

 actual measurements range from 002 to 0-05 mm. 



This simple formation of the teeth in the gizzard of 

 Petalura, is of great phylogenetic interest and importance. 

 The gizzards of the larvse of all the more specialised 

 Anisoj)tera possess only four sets of teeth ; each set being 

 developed on an elongated oval layer of chitin, and the 

 whole set being spoken of as a "fold" or "field." In 

 Fetalura, we have no actual chitinised fields, but only a slight 

 development of chitin round each tooth. This formation 

 can be easily understood from the enlarged diagrams of some 

 of the separate sets of teeth in gizzard No. 2. From this we 

 are able to conclude that the four fields of the more special- 

 ised Anisojjtera have been formed by the merging, in pairs, 

 of eight simpler fields : that the teeth were developed as 

 chitinous protuberances, on the gizzard wall itself ; and that 

 the chitinous oval field was formed later on by the extension 

 of the chitin at the base of each tooth, so as to include finally 

 the whole set of teeth. In the Petalura gizzard we can see 

 the beginning of the formation of a chitinous field, if we 

 look at the enlarged figures of the teeth of the seventh and 



