Mr. G. H. K. Thwaites on Conjugation in the Diatomacese. 9 



three thoracic segments therefore are here blended with the head, 

 but we likewise find the cephalic region supporting a pair of true 

 locomotive feet, which are the first pair of abdominal appendages ; 

 the first abdominal ring therefore is also in the present instance 

 inseparably united with the head. The next two distinct rings 

 are the second and third of the abdomen, each with its pair 

 of natatory feet ; the fourth abdominal ring carries also a pair of 

 natatory feet, but becomes indistinct at its posterior margin. 

 Here commences "the great ovarian chamber, behind which is the 

 posterior region of the abdomen with its five distinct rings and 

 terminal setigerous appendages. 



EXPLANATION OF PLATES I., II. 

 Plate I. 



NOTODELPHYS ASCIDICOLA. 



Fig. 1. Notodelphys ascidicola magnified and viewed from above. 



Fig. 2. The same viewed in profile. 



Fig. 3. Accessory oral appendage of the first pair. 



Fig. 4. Labrum. 



Fig. 5. Mandible. 



Fig. 6. Accessory oral appendage of the second pair (maxilla of the first 

 pair). 



Fig. 7. Accessory oral appendage of the third pair (maxilla of the second 

 pair). 



Fig. 8. Accessory oral appendage of the fourth pair. 



Fig. 9. Accessory oral appendage of the fifth pair. 



Fig. 10. One of the natatory feet. 



Fig. 11. Ovigerous chamber viewed from below in order to show the aper- 

 ture for the escape of the ova. 



Fig. 12. One of the immature phases of N. ascidicola. 



Fig. 13. Antenna of this phase. 



Fig. 14. N. ascidicola in a still earlier stage of development. 



Plate II. 



Fig. 15. The same. viewed from above. 



Fig. 16. Its labrum. 



Fig. 1 7. One of its mandibles. 



Fig. 18. One of the appendages immediately succeeding the antennae. 



Fig. 19. One of the penultimate pair of accessory oral appendages. 



Fig. 20. One of the last pair of accessory oral appendages. 



Fig. 21. A pair of true ftet in this stage. 



Fig. 22. Larva as it appears on escaping from the ova of fig. 1 2. 



II. — On Conjugation in the Diatomacese. By G. H. K. Thwaites, 

 Lecturer on Botany and Vegetable Physiology at the Bristol 

 Medical School. 



[With a Plate.] 



To the Editors of the Annals of Natural History. 

 GENTLEMEN, 2 King3down Parade, Bristol, May 11th, 1847. 



It gives me great pleasure to be able to announce through the 

 medium of your valuable journal the very interesting discovery 



