Figure V. Explanalion : t. Siliqua patula Dixon. Mature spermatozoon, 

 July 22nd. h, head ; nip, middlepiece ; f, llagellum. Length of head and 

 middlepiece .004 mm. 2. Siliqua patula Dixon. Mature ovum, July 

 22nd. n, nucleus; nc, nucleolus. Diameter 0.125 nnn. 3. Siliqu.'i patula 

 Dixon. Section of follicle of spermary, July 22nd. Greatest lengtli 

 0.6 mm. 4. Schizothaerus nuttalli Conrad. Mature spermatozoon, Marcli 

 30th. Letter reference as in i. Length of head and middlepiece 

 .005 mm. 5. vSchizothaerus nuttalli Conrad. Section of follicle of sper- 

 mary, March jotli. Greatest length 0.6 mm. 6. Schizothaerus nuttalli 

 Conrad. Section of follicles of ovary almost spawned out, Marcli 30th. 

 7. Paphia staminea (Conrad). Section of follicle of spermary, March 

 30th. Greatest length 0.5 mm. 8. P^aphia staminea (Conrad). Ovum 

 approaching maturity, Marcli 30tli. Letter reference as in 2. Diam- 

 eter 0.12 mm. Q. Paphia staminea (Conrad). Mature spermatozoon, 

 March 30th. Letter reference as in i. Length of head and middlepiece 

 .008 mm. 10. Mya arenaria Linnaeus. Mature spermatozoon, Septem- 

 ber 4tli. Letter reference as in i. Length of head and middlepiece 

 .004 mm. II. Mya arenaria Linnaeus. Developing phases of spermato- 

 cytes from a group of four cells to a small follicle of many cells. The 

 earlier stages more highly magnified than tlie latter. 12. Mya arenaria 

 Linnaeus. Mature ovum, Sei)teinber 4th. Letter reference as in 2. 

 Diameter 0.12 mm. 13. Cardium corbis Martyn. Hermaphroditic con- 

 dition shown. Follicles of spermary surrounded liy follicles of ovary. 

 August 29th. Greatest length of sperm follicle 0.4 mm. 14. Cardium 

 corbis Martyn. Mature spermatozoon, August 28th. Letter reference 

 as in I. Len.gth of head and middlepiece .008 mm. 



Figure VI. Explanalion. i. Pholachdea penita (Conrad). Fidl\ (le\ eloped 

 spermatozoon, March 27th. Ii, head ; nip, middlepiece ; f. tlagellum. 

 Length of head and middlepiece .005 mm. 2. Pholadidea penita (Con- 

 rad). Section of follicle of spermary, March 27th. Greatest length 

 0.6 mm. 3. Pholadidea penita (Conrad). Section of follicle of ovary. 

 March 27th. Greatest length 0.5 mm. 4. Saxidomus giganteus Des- 

 hayes. Fully formed but not active spermatozoon, November 2gth. Let- 

 ter reference as in i. Length of head and middlepiece .008 mm. 5. Sax- 

 idomus giganteus Deshayes . Section of follicle of spermary, November 

 29th. Greatest length 0.65 mm. 6. Saxidomus giganteus Desliayes. 

 Ovum enclosed in a dense membrane and developing from the wall of a 

 follicle, November 29th. f, follicle ; m, membrane ; n, nucleus ; nc, 

 nucleolus. Long diameter of ovum 0.125 mm. 7. Saxidomus giganteus 

 Deshayes. Ovum approaching maturity, June i2tli. Letter reference as 

 in 6. Diameter of ovum 0.125 mm. 8. Macoma nasuta Conrad. Mature 

 spermatozoon, March 31st. Letter reference as in i. Length of head 

 and middlepiece .006 mm. 9. Macoma nasuta Conrad. Section of fol- 

 licle of ovary, March 31st. Greatest length 0.5 mm. 10. Mytilus edulis 

 Linnaeus. Section of follicle of spermary from mantle, March 25th. 

 Greatest length 0.4 mm. 11. Mytilus edulis Linnaeus. Fully formed 

 spermatozoon, March 25th. Letter reference as in i. Lenglii of head 

 and middlepiece .005 mm. 12. Mytilus edulis Linnaeus. Section of fol- 

 licle of ovary from mantle, INIarch 25th. Greatest length 0.4 mm. 



