KONGL. SV. VET. AKADEMIENS IIANDL. HAND. 19. N:0 7. 11 



the perisoiue alone. It is this that has led me to inquire whether in Endocyclic Gnatho- 

 stomes, Avhere the primitive dorso-central ossicle is broken up in consequence of the 

 eruption of the excretory opening, or, as in the case of Salenia, where a periproct is 

 formed more or less outside that ossicle, the antil membrane which is substituted for 

 the removed parts, may not belong to the perisomatic system, continuous under the 

 calycinal as well as under the ambulacral system, and thus forming by itself the 

 whole of the skeletal säck. 



Being thus the principal constituent of the exteriör of the Echinoidean skeleton, 

 the perisome has a tendency to assert its supremacy, so to speak, by intervening 

 between the tAvo other, or even infringing upon them. I have shoAvn hoAv largely 

 this takes place among the Star-fishes. ^) In the young of Asterias glacialis the caly- 

 cinal system, originally compact and complete in all its parts: central ossicle, five 

 costals and five radials, is broken up OAving to the predominant development of the 

 perisome, by dint of which the central ossicle is replaced by an anal membrane, the 

 five costals severed from one another almost past recovery, and the five radials 

 moved far off to the tips of the rays. And among the Echinoidea, Avhen in the 

 Collyrites the ambulacra of the bivium are seen diverging from their normal position, 

 the space left open is found to be filled by expanded interradials. Wherever there 

 is a question of thoroughly making out the relations of the three systems cora- 

 bined in the Echinodermal skeleton, it is of primary importance to keep in vieAv this 

 ascendency of the perisomatic system, in \'irtue of Avhich it is seen in more than one 

 Avay, and often in a delusory manner, to mix itself Avith the other systems, and ob- 

 scure their relations. 



The Perischo-echinida are characterised by the presence in eacli interradium, at 

 the ambitus, of more than tAvo series of perisomatic plates, out of Avhich the ad- 

 ambulacral alone attain to the peristome and the calycinal system. It Avas believed 

 that this mode of structure Avas limited to the Paleozoic era, and that Avith the single 

 exception of the genus Tetracidaris Cotteau, of the Cretacean era, all the Mesozoic, 

 Tertiary and existing Echinoidea had only tAvo series of plates in each interradium. 

 To this another exception has lately been added. During the latter part of the Tri- 

 assic period the sea that covered Southern Tyrol Avas inhabited by a fauna, preserved 

 at S:t Cassian, ^) in AAdiich survivors from the Pateozoic era, such as Spirigera, Cyrtina, 

 Retzia and Murchisonia, Avere coeval Avith numerous Mesozoic and modern types, then, 

 as far as Ave knoAv, appearing for the first time, and species of Orthoceras lingered 

 among the earliest Ammonites, of an antique aspect. In this assemblage of old and 

 neAv the Echinoidea were represented by numerous forms referable to Cidaris and a 

 single Hypodiadema, and along Avith these by the very singular little Tiarechinus 

 princeps (Laube) lately described by Neumayr. ^) I am indebted to D:r Franz v. 

 Hauer, and D:r Dionys Stur, of Vienna, for the opportunity kindly offered to exa- 



') Études, p. 86, pl. LIII, tig. 256 — 260. 



-) Laube, die Fauna der Schichten von S:t Cassian, Wiener Denkschriften, XXIV, XXV. XXVIII, XXX, 



1868. 

 ä) Sitzungs-Berichte d. K. Akad. d. Wiss. Wien, Abtli. I, LXXXIV, Juni 1881. 



