April, 1920] 



The Canadian Field-Naturalist 



71 



species. It appears to be intermediate between the 

 tv/o genera, but bears in most cases the Httle cupped 

 beaks distinctive of Musculium. 



Fig. 3. ^Nfusculium transversum. 



M. transversum abounds in the Rideau Canal, 

 along the right bank, immediately above the by- 

 wash at Hartwell's Locks, and in the by-wash it- 

 self. This is the only Sphaerium or Musculium I 

 have noticed until the discharge is reached, when 

 a few 5. simile may be found. Lower down the 

 river, it is not uncommon in the rapids near Billing's 

 Bridge. In the Ottawa I have found it along both 

 shores of the lower third of Duck Island. It seems 

 to prefer mud to sand in that locality, and com- 

 paratively quiet waters; but it withstands strong 

 currents over clay in the by-wash, and over coarse 

 gravel in the Rideau River. 



The number of shells of this species disclosed at 

 Hartwell's Locks when the canal is unwatered is 

 really phenomenal. Nearly all must perish annu- 

 ally, but in many successive summers no lessening 

 of the multitude had been observed. 



20. MUSCULUM TRUNCATUM Linsley. This 

 shell was first noticed in Nepean Bay, near the 

 Broad Street Railway Station. It has since been 

 found in many other localities. In fact it is a very 

 common shell on the Ontario side of the Ottawa, 

 and in the Ottawa itself, at Duck Island, below he 

 sand bars. The Duck Island shells (No. 2371 of 

 my collection) are thought by Dr. Sterki to be 

 "possibly distinct." In Nepean it occurs in great 

 profusion in a wayside pool four or five miles south 

 of Britannia, and west of the road between th? 

 third and fourth concessions fronting on the Rideau. 

 East and West of Britannia village it is to be found 

 in early summer in ponds formed by the overflow 

 of the river, and to the south of the Grand Trunk 

 Railway, in the "hole in the hill " on the the Honey- 

 well farm. Later in ordinary seasons, all these 

 ponds become dry; but year after year no diminu- 

 it!on in the numbers of these and other species has 

 been observed. Many shells must surpive becaus'' 

 either deeply buried, or like S. occidentale immune 

 to dessication. 



I have not found the shell in the Province of Que- 

 bec, though it doubtless occurs there in suitable 

 localities. Dr. Dall (Harr. Rxpd. Vol. 13. p. 140) 



ucords it as occuring in M.;»hy Lake, Athabasca. 

 Oi»e of the Richardson sphaeria from the same lo- 

 cality is probably M. Iruncatum. 



M. Iruncatum is very thin, pellucid, and but 

 slightly inflated, the posterior margin is longer in a 

 straight line than that of any other shell of the 

 genus. Exteriorly the shell is sraw coloured; in- 

 teriorly a very pale blue. The average size is 

 13.2 X 11.15 X 4.5 mm., or 100:87:56. 



The anatomy of M. Iruncatum has been most 

 carefully worked out by Mr. Ralph J. Gilmore of 

 Cornell University: Nautilus, Vol. 31 p. 16 e/ seq. 

 His figure, which may be regarded as applicable lo 

 the entire genus, I am enabled to reproduce through 

 the courtesy of Dr. Walker. The margins, anterior 

 and posterior, of the shells ordinarily found near 

 Ottawa are much less rounded than those of the 

 shell figured. 



Fig. 4. Musculium Iruncatum Linsley. 



M.— Mouth. 



L;). — Labial palp.s. 



Ks. — Ofsopliag^us. 



Lv. — Liver. 



Sto. — Stomach. 



Int. — Inte.stine. 



R. — Rectum. 



A. — Anus. 



Cb. G. — Cerebral ganglion. 



Ps. G. — Parieto Splanchnic gaglion. 



P.G. — Pedal ganglion. 



Sta.— Statocyst. 



V. — Ventricle. 



All. — Auricle. 



K.O. — Kidney opening. 



T.— Sperm follicles. 



C). — Kgg follicles. 



G.I). — Genital opening. 



AA. — Ant. adductor muscle. 



I'd. — Po?t. adductor muscle. 



Ft. — P\)ot. 



(,'1 Ch. — Cloacal chamber. 



Kx. S. — K.xcurrent siidioii. 



Int. S. — Incurreni siphon. 



G.— Gill. 



Man. — Mantle. 



Shi. -Shell. 



Kd. — Kidney. 



P. — Pericardium. 



(To be continued). 



