260 PROFESSOR OSKAR CARLGREN ON THE GENUS PORPONIA 



order from and with the third, a suppression which probably stands in connection with 

 the animal's mode of life. 



In this hypothetical explanation of the arrangement of the Rugosa septa, I have 

 mainly intended to direct attention of palaeontologists to the fact that the Rugosa type 

 may be explained in various ways ; moreover, a closer examination of Rugosa has 

 shown that the development is different in different genera, that some of them retain a 

 bilateral and others a more radial arrangement ; the development of septa in this 

 group, however, requires further examination. 



The hypothesis put forward by me with regard to the origin of Rugosa seems to me 

 to speak for itself. The presence of the 4 growth-zones after the development of the 

 first 12 mesenteries in Rugosa may be fully explained by supposing a development of 

 certain mesenteries like those in the Endocoelactidse. The enlargement of the 4 

 primary lateral endoccels has led to developmental zones for the new mesenteries being 

 removed to these areas instead of to the 6 primary exoccels. The origin of a 4 (8)- 

 rayed type in certain Rugosa may be explained in this way. In any case, I consider 

 the above explanation as good as, if not better than, that put forward by DUERDEN, to 

 the effect that Rugosa must stand in a certain relation to the Zoantharia (Zoanthidse). 

 In consequence of this view, he also considers the latter group as very old, a view, 

 however, I have some difficulty in accepting, as the Zoanthidas are obviously rich in 

 species, and presumably form a group which is still in process of differentiation. See 

 also my work in BRONN. 



Finally, it seems convenient further to characterise the family Endoccelactidse with 

 the genera Halcurias. Porponia, and Syrihalcurias. 



Family Endocoelactidiv. 



Atheuaria (Protactininae ?) with thick, sometimes cartilaginous body-wall, without 

 sphincter and fossa, with spirocysts in the ectoderm of the body-wall and oesophagus. 

 Arrangement of the mesenteries quite different from that of the normal Actiniaria type, 

 owing to the development of the second and third order of mesenteries in the endocoels. 

 In consequence, the arrangement of the tentacles very different from the normal type 

 (among others, 1 tentacles of the first order immediately border on those of the second 

 order). Sexual organs present on all the stronger mesenteries from and with those of 

 the first order, including directive mesenteries. 



Genus Halcurias M'~M.m-ric}\ = Endoccelactis Carlgren. 



Endocoelactidaa with ca. 68 mesenteries, 36 of which are perfect. Four cycles of 

 mesenteries. The mesenteries of the fourth order regularly arranged on each side of 

 those of the third order, the mesenteries of the same pair equally developed. The 

 perfect mesenteries arranged as follows : 20 (6 + 4 pairs) + 16 (8 pairs), of which the 

 first 20 str6ng, extending over the whole length of the body ; the others only developed 

 in the distal part, and weak. The body cylindrical. The tentacles not bridge-like 



(ROY. soc. EDIN. TRANS., VOL. L., 68.) 



