NoTT. — On N.Z. Composite Ascidians. 327 



Except the colom% no one of the above characters can be 

 considered such as is not accountable for by simple variation. 

 It has been seen that the calcareous elements are capricious 

 in their distribution, if the term be appropriate to anything 

 which we surmise to have a cause but cannot explain. 



Paet III. — Systematic and Geneead. 



It now remains for me very briefly to point out in a 

 concluding summary the features of systematic or structural 

 importance which the preceding descriptions appear to suggest. 



Every authority upon the subject concedes the difficulty 

 of exact classification : hence we need not be astonished to find 

 that each new form discovered is hard to place systematically. 

 This difficulty meets us at the outset in the forms which I 

 assign to Milne-Edwards's genus Leptoclinum. Herdman 

 (" Challenger " Eep., vol. xiv., pt. xxxviii., p. 260) sums up the 

 features of importance in distinguishing the genera Didemnum 

 (Savigny, 1816) and Leptoclimim (1811) as follows : — 



Didemnum : Colonies thick ; atrial siphon placed far back, 

 with no languet ; three rows of stigmata. 



Leptoclinum : Colonies thin ; atrial pore further forward, 

 with a languet ; four rows of stigmata. 



Now, with one exception, our forms have four rows of stig- 

 mata, and are very thin ; but no languets were noted, the 

 atrial pore being placed in the middle of the thorax. The one 

 exception, however (L. tuhcratum) , can scarcely be regarded 

 as definitely settled owing to the decalcified specimens becom- 

 ing much contorted in sectioning : fairly-advanced young forms 

 always exhibit four rows of equally well developed stigmata. 

 The thickness of a colony seems very unsatisfactory as a test 

 of generic affinity, but, if it is to be regarded as sound, I think 

 we must place our four Auckland species among the Leptocli- 

 nids. My own personal convictions are strongly against the 

 generic distinctness of Milne-Edwards's group, and, as the 

 four species here noted furnish a strong bond of connection 

 between it and the older genus Didemnum, I should prefer 

 to amalgamate the two genera under Savigny's genus. 



The question of affinity to other species is also one of great 

 difficulty. I have only the "Challenger" species to guide 

 me, and these, owing probably to the nature of his material, 

 are inadequately described by Herdman. Thus, in spite of 

 the importance he attaches to the atrial pore in classification, 

 in only one species (L. japonicum) does he mention it in his 

 descriptions, and there he says, " The atrial siphon [sic'] is 

 placed on the dorsal edge of the thorax half-way down"! 

 Further, he seldom mentions the alimentary tract except very 

 briefly, and has not noticed the peculiar oesophageal con- 



