Miscellaneous. 479 



a vain effort of the limited faculties of a finite nature. Yet nearly- 

 all these periods have come and gone since the reptilian animals 

 played their parts in the triassic and perniian worlds. 



" It is hard to realize the surpassing interest with which the evi- 

 dences of such ancient life are first received, scrutinized, and com- 

 pared, and by which is lightened the labour of gaining ideas of the 

 frames, the limbs, the weapons, and ways of life of these long-since 

 perished animals. Unlike the poet, and dealing -with denser elements, 

 the geologist nevertheless, but with eyes fixed and gaze intent, ' bodies 

 forth the forms of things unknown,' ' turns them to shapes,' and, in 

 the transitory continents which successively come into and fade away 

 from his field of vision, gives to them ' a local habitation and a 

 name.' " 



MISCELLANEOUS. 



Note on the Emhryogemi of the Tuaicata of the Group Lucise. 

 By M. A. GiARD, 



I HAVE repeatedly insisted * upon the necessity that exists for 

 separating clearly the compound Ascidia of the group Didemnidae 

 from other forms belonging to a very different type, of which I have 

 made the family Diplosomidse. Besides important anatomical and 

 embryogenical differences, the presence of numerous calcareous 

 spicules in the tunic of the Didemnidae is a practical character which 

 enables them to be easily distinguished from the Diplosomidte, in 

 which these spicules are replaced by pigment-granules. 



This new family includes: — 1, the genus Dijjlosoma, MacDonald ; 

 2, the genus Pseudodidemnum, containing a great number of 

 species, specially Didemnum gelatinosinn, M.-Edw., Leptodimim 

 gelatinosum,'hl.-YidiW. {Polyolinum, Lister), the Lissoelina of Verrill, 

 &c. ; 3, the genus Astellium., including many new species, one of 

 which, no doubt, answers to Leptodinum punctatum, Forbes. 



The Ascidian so well investigated by Kowalevsky under the name 

 of Didemnum sti/Jiferum f appears to be intermediate between the 

 genera Diplosoma and AsteUimn. 



The species that I have taken as the type of the latter genus, 

 Astellium spongiforme, first found on the coast of Brittany, is also 

 common at Saint- Vaast-la-Hougue in Normandy and on the shores 

 of the Boulonnais. I have this summer undertaken some fresh 

 investigations upon the curious embryogeny of this Ascidian ; the 

 results at which I have arrived, brought together with those of the 

 magnificent work of Kowalevsky on the embryogeny of Pyrosoma %, 

 seem to me to throw an unexpected light upon the relations of the 

 Diplosomidae with the other Tunicata. 



* Archives de Zoologie, tomes i. & ii. 1872 and 1873. 

 t Schultze's ' Archiv fiir mikr. Anat.' Bd. x. 1874. 

 \ Schultze's 'Archiv/ Bd. xi. 1875. 



