ECHINOIDEA. II. 



and even to the Echinidce and Echinometrida they seem to show affinities, viz. through their large 

 headed (tridentate) pedicellarise (Op. cit. p. 132). - In the Panamic Deep-Sea Ech. p. 150 Professor 

 Agassiz finds it interesting to trace the changes between Pourtalesia proper with its bottle-shaped 

 outline, deeply sunken actinal and anal grooves, its well developed anal proboscis, and such a type 

 as Plexechinus, in which the Pourtalesian features have almost disappeared, to pass into a more An- 

 anchytid type, represented by Urechinus and Cystechinus. In the further development the rudimentary 

 phyllodes and labium become specialized in Genicopatagus, Argopatagus and Homolampas. Next An- 

 anchytid petals like those of Paleopneustes, Linopneustes lead us gradually to the petaloid type of 

 the recent Spatangoids. On p. 173 it is stated for Argopatagus that the fact that the second plates 

 of the posterior zone of the posterior lateral ambulacra almost separate the labium from the sternum 

 as in Plexechinus is an indication of the affinities of the genus to the Pourtalesiae. 



Agassiz thus evidently seems to consider the Pourtalesiee as the centre from which all the 

 other Irregular Echinoids have developed; that the group itself has developed from one of those 

 named does not seem to be the meaning of the famous Echinologist -- the Pourtalesiae are evidently 

 regarded as embryonic forms, which have given rise to all the different groups, to which the affi- 

 nities are pointed out, since the affinities probably must mean real genetic relationship. I think I 

 need not here point out in a more detailed manner that the more prominent characters of the Pour- 

 talesiae are highly specialized, not at all embryonic. But Professor Agassiz does not seem to take 

 into consideration that the different characters are not of the same value; structural characters of 

 the highest systematic importance and irrelevant, vague resemblances are regarded as equivalent 

 criteria of relationship. (Comp. my remarks on this theme in the Echinoidea of the Danish Siam- 

 Exped. p. 50.) 



Also Urechinus naresianus is held by Agassiz (Blake-Ech. p. 52) to be a representative of 

 the oldest Spatangids, ^leading us little by little to Spatangoid genera in which the ambulacra become 

 more or less petaloid, as in Homolampas, Paleopneustes and the like, till we get the modern type of 

 Spatangus proper, with well defined petaloid ambulacra and a highly developed subanal fasciole* etc. 

 It is evident that the quite rudimentary abactinal tube-feet and pores in Urechinus is a highly speci- 

 alized feature, which may possibly give rise to further stages in which these tube-feet and pores com- 

 pletely disappear; but it is rather inconceivable how these rudimentary pores and tube-feet, which 

 doubtless represent a reduction from the more primitive condition, where the pores were double and 

 the tube-feet well developed, should again give rise to petaloid structures with large, double pores 

 and well developed tube-feet. Also the fascicles have doubtless developed separately in several groups 

 - in the same manner as the polyporous condition of the ambulacra among the Echinina. The 

 same objections may be made against regarding Calymne as holding an intermediate position between 

 the Pourtalesiae proper and such genera as Paleopneustes and Palseotropus, and against finding in 

 Cystechinus (Urechinus), Pourtalesia and the allied genera Palaotropus, Ncolampas and the like 

 a proof of the affinities of the Spatangoids with the Echinolampadae*. (Chall.-Ech. p. 148). - - Upon 

 the whole I cannot join Professor Agassiz when expressing his joy of how the structure of so many 

 of the Spatangoid forms is satisfactorily explained by the different genera of Pourtalesia 1 collected by 

 the Challengers and how greatly the knowledge of the members of this family has helped us to 



