OF ARTS AND SCIENCES. 45 



ascogonium, unmistakably present as Woroniu's hypba, perishes with- 

 out taking part morphologically in the formation of asci." This would 

 account for the presence of the larger cells in the primordial tissue, 

 and their absence later, but I must confess that such an explanation 

 seems to me decidedly strained, though perhaps no more so than in 

 the case of Xylaria already mentioned. 



The absence of a trichogyne in Hydrothyria is to my mind as cer- 

 tain as in the other lichens studied, although in all cases the statement 

 rests upon a purely negative proof. If we crush thin sections of an 

 apothecium in which the asci are still young, (the same treatment being 

 employed as formerly,) the elements of the hyraenium may be separated 

 easily. The paraphyses then appear as delicate, filamentous threads, 

 which are either simple or copiously branched, and frequently exhibit a 

 form of anastomosis by means of a bridge-like connection. (Plate IV. 

 Figs. 28-30.) Both asci and paraphyses arise from the same vegeta- 

 tive cells of the subhymenial layer, and exhibit the same mutual rela- 

 tionship as in the other forms studied. (Plate IV. Figs. 26, 27.) 



In conclusion, then, Hydrothyria can be regarded in no sense as a 

 transitional form between the true foliaceous lichens of the Pannaria 

 type and the gelatinous Collemaceae. The presence of a cortex in- 

 vesting both surfaces of the thallus, the non-gelatinous character of the 

 thalline tissue, and the Scytonemoid gonidia, are facts which taken to- 

 gether would give to the genus an anomalous position in the neighbor- 

 hood of Peltigera and Pannaria. If such a transitional form does 

 occur, it must rather be looked for in the genera Pannaria and Physma, 

 where the Scytonema type of gonidia has given place to the Nostoc 

 type, and where the dense but large-celled hyphal tissue of Heppia, in 

 which the hyphne give the character to the thallus, has, by the necessity 

 arising from a decidedly gelatinous host, become modified, so that now 

 the gelatinous membranes of the algae preponderate, and take an equal 

 or greater share with the hyphfe in impressing a definite character 

 upon the thalline structure. Furthermore, as already noted by Tuck- 

 erman,* no one can examine the two species of Physma without being 

 at once struck by the remarkable similarity in thalline structure, on 

 the one hand, between Physma luridum and Pannaria rubiginosa, and, 

 on the other, between Physma hyrsceum, (Mass.) Afzel., and the typical 

 Collemacere. (Cf. Plate VI. Fig. 35, and Plate III. Fig. 22. Also 

 Plate VI. Fig. 34, and Plate VII. Fig. 36.) 



At this late date it hardly seems necessary to dwell at much length 

 upon the method of reproduction of the true Collemacea?. The clear- 



* Tuckerman, Genera Lichenum, p. 57. 



