Figure V. Explanation: 1. Siliqua patula Dixon. Mature spermatozoon, 
July 22nd. h, head; mp, middlepiece; f, flagellum. Length of head and 
middlepiece .004 mm. 2. Siliqua patula Dixon. Mature ovum, July 
2end. n, nucleus; nc, nucleolus. Diameter 0.125 mm. 3. Siliqua patula 
Dixon. Section of follicle of spermary, July 22nd. Greatest length 
0.6mm. 4. Schizothaerus nuttalli Conrad. Mature spermatozoon, March 
30th. Letter reference as in 1. Length of head and middlepiece 
005 mm. 5. Schizothaerus nuttalli Conrad. Section of follicle of sper- 
mary, March 30th. Greatest length 0.6 mm. 6. Schizothaerus nuttalli 
Conrad. Section of follicles of ovary almost spawned out, March 3oth. 
7. Paphia staminea (Conrad). Section of follicle of spermary, March 
30th. Greatest length 0.5 mm. 8&8. Paphia staminea (Conrad). Ovum 
approaching maturity, March 30th. Letter reference as in 2. Diam- 
eter 0.12 mm. 9. Paphia staminea (Conrad). Mature spermatozoon, 
March 30th. Letter reference as in 1. Length of head and middlepiece 
008 mm. 10. Mya arenaria Linnaeus. Mature spermatozoon, Septem- 
ber 4th. Letter reference as in 1. Length of head and middlepiece 
004 mm. tt. 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 the latter. 12. Mya arenaria 
Linnaeus. Mature ovum, September 4th. Letter reference as in 2. 
Diameter 0.12 mm. 13. Cardium corbis Martyn. Hermaphroditie con- 
dition shown. Follicles of spermary surrounded by follicles of ovary. 
August 20th. Greatest length of sperm follicle 0.4 mm. 14. Cardium 
corbis Martyn. Mature spermatozoon, August 28th. Letter reference 
as in t. Length of head and middlepiece .oo8 mm. 
Figure VI. Explanation. 1. Pholadidea penita (Conrad). Fully developed 
spermatozoon, March 27th. h, head: mp, middlepiece; f, flagellum. 
Length of head and middlepiece .o95 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 20th. Let- 
ter reference as in 1. Length of head and middlepiece .oo8 mm. 5. Sax- 
idomus giganteus Deshayes . Section of follicle of spermary, November 
20th. Greatest length 0.65 mm. 6. Saxidomus giganteus Deshayes. 
Ovum enclosed in a dense membrane and developing from the wall of a 
follicle, November 20th. f, follicle; m, membrane; n, nucleus; ne, 
nucleolus. Long diameter of ovum 0.125 mm. 7. Saxidomus giganteus 
Deshayes. Ovum approaching maturity, June 12th. Letter reference as 
in 6. Diameter of ovum 0.125 mm. & Macoma nasuta Conrad. Mature 
spermatozoon, March 31st. Letter reference as in I. Length of head 
and middlepiece .oo6 mm. 9. Macoma nasuta Conrad. Section of fol- 
licle of ovary, March gist. Greatest length 0.5 mm. to. Mytilus edulis 
Linnaeus. Section of follicle of spermary from mantle, March 25th. 
Greatest length 0.4 mm. tr. Mytilus edulis Linnaeus. Fully formed 
spermatozoon, March 25th. Letter reference as in 1. Length of head 
and middlepiece .co5 mm. 12. Mytilus edulis Linnaeus. Section of fol- 
licle of ovary from mantle, March 25th. Greatest length 0.4 mm. 
