304 F. F. LAIDLAW. 



It is obvious that PL langii differs more from any one of the other Planucera than they 

 differ amongst themselves. The most pronounced differences are of course for the male apparatus 

 the presence of paired vesiculae seminales, for the female the character of the bursa copulatrix, 

 which appears to develope as an outgrowth from the vagina. I am inclined to suppose that an 

 organ of a similar character occurs in Veri-ill's genus Heterostylochus [15]. In describing it he 

 says of the female organs that, " a long narrow median duct runs far forward from the female 

 orifice and expands into a flask-shaped seminal receptacle or spermatheca near the male organs; 

 a swollen egg-duct also connects with the female orifice." The spermatheca may perhaps be 

 the same organ that I have called the bursa copulatrix. Its true character may be judged 

 from its appearance as shown in PI. XV. fig. 13. 



Thysanozoon and Thysanoplana. Von Stummer-Traunfels [14] has recently suggested 

 a modified definition of the genus Thysanozoon Grtibe : " Pseudoceriden mit spitzohrahnlichen 

 faltenfbrmigen Randtentakeln, mit zottenfdrmigen dorsalen Anhangen in welche bisweilen Diver- 

 tikel der Darmaste hineintreten, ohne jedoch daselbst auszumtinden : mit einfach oder doppelten 

 mannlichen Begattungsapparate." 



The genus is thus made to include a number of tropical species, several new species being 

 described in an interesting review of the genus by this author, who has also given a series 

 of beautiful figures of most of the species. He finds that of all the species known to him only 

 one, viz. Thysanozoon brncchii, has the dorsal papillae invaded by gut-diverticula. Thysanozoon 

 brocchii and about six other species have paired male apparatus. Th. auropunctatus (Kel. Coll.) 

 and several others have an unpaired penis. 



Von Plehn [10] has described a new genus, Thysanoplana, containing two species evidently 

 closely allied to Thysanozoon. It is separated from this genus on the following grounds. Firstly, 

 " Im Gegensatz zu Thysanozoon der mannliche Apparat ist unpaar." Secondly, " Hauptdarm mit 

 zahlreichen vielfach tibereinander entspringenden Darmastwurzeln." The papillae are stated 

 definitely to be invaded by gut-diverticula. 



Now if we accept von Stummer-Traunfels' amended definition of the genus Thysanozoon, 

 it is evident that the first of the characters, viz. the unpaired male apparatus, will not serve 

 to distinguish Thysanoplana fi-om Thysanozoon, and as to the second I have found that a similar 

 peculiarity, though much less marked, occurs in Thysanozoon plehni (mihi), whilst traces of the 

 same occur even in Thysanozoon brocchii (see above). Accordingly in this case the generic 

 name Thysanoplana cannot, I think, be maintained. 



On the other hand, if we do not accept the amended definition and retain Thysanoplana, 

 it will be necessary to subdivide Thysanozoon into several genera as follows : — 



A. Species with gut-diverticula in papillae. 



a. Penis unpaired {Thysanoplana). 



b. Penis paired {Thysanozoon brocchii). 



B. Species without gut-diverticula. 



a. Penis unpaired {Thysanozoon plehni, etc.). 



b. Penis paired {Thysanozoon, sp.). 



The mode of origin of the gut-diverticula cannot, I consider, be employed as a generic 

 character. On the whole it is simplest at present to refer all the described species to the 



