116 



P. B. SIVICKIS. 



monly regarded as the most trustworthy criterion of specific differ- 

 ences, sections of this region of sexually mature animals have been 

 made. 



b.T* 



Figs. 3, 4. Atrial genital complex of the two species of Planaria: Fig. 3, 

 P. lata; Fig. 4, P. maculata combined from two adjoining sections; c, copula- 

 tory organ ; /, female region of atrium ; g, circular groove or furrow on copu- 

 latory organ ; in, male region of atrium ; v, valve or fold between the two 

 parts of atrium. 



Fig. 3 is a median sagittal section through this region of the 

 mid-western form and Fig. 4 a similar section from P. maculata, 

 both from sexually mature animals. The circular groove (g) 

 about the apical region of the copulatory organ (c) is deeper in 

 P. lata and the outline of the fold or valve (v) between the com- 

 mon portion of the atrium and the female duct (/) is very differ- 

 ent in the two species. Other differences in shape of the parts 

 and cavities are evident, but may be due in part to differences in 

 muscular contraction. The differences in the atrial complex con- 

 firm the conclusion that the two forms are different species, and 

 since the mid-western form does not agree with descriptions of 

 other species already given, it is evidently an unnamed species and 

 is named and described as follows : 



Planaria lata n. sp. — Length of full-grown, sexually mature in- 



