KONGL. SV. VET. AKADEMIENS HANDL. BAND. 19. N:0 7. 11 



the perisonic alone. It i.s this that has led lue to inquire whethei' in Endocyclic Gnatlio- 

 stoines, where 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 anal meml)rane Avhich is substituted for 

 the removed parts, may not l)eh)ng to the perisomatic system, continuous under the 

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

 whole of the skeletal sack. 



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

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

 between the two other, or even infringing upon them. I have shown how largely 

 this takes place among tlie 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 owing to the predominant development of the 

 j)erisome, 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, when 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 tlioroughly making out the relations of the three systems com- 

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

 ascendency of the perisomatic system, in virtue of which it is seen in more than one 

 way, and often in a delusory manner, to mix itself with the otlier systems, and ob- 

 scure their relations. 



The Periseho-echinida are characterised by the presence in each interradium, at 

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

 ambulacral alone attain to the peristome and the calj'cinal system. It was believed 

 that this mode of structure was limited to the Paleozoic era, and that with the single 

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

 Tertiary and existing Echinoidea had only two series of ])lates 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 Tjrol was inhabited by a fauna, preserved 

 at S:t Cassian, "'') in which survivors from the Pala30zoic era, such as Spirigera, Cyrtina, 

 Retzia and Murchisonia, were coeval with numerous Mesozoic and modern types, then, 

 as far as we know, appearing for the first time, and species of Orthoceras lingered 

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

 new the Echinoidea were repi'esented by numerous forms referable to Cidaris and a 

 single Hypodiadema, and along with these by the very singular little Tiarechiiuis 

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

 Hauer, and D:r Dionts Stur, of Vienna, for the opportunity kindly oft'ered to exa- 



1) Etudes, p. 86, pi. LIII, fig. 256-260. 



-) Laube, die Fauna der Schichten von S:t C:assi;\n, VVieiier Deiikscluifti ii, XXIV, XXV. XXVIII, XXX, 



1868. 

 3) Sitzutigs-Berichte d. K. Akad. d. Wiss. \Vien, Abtb. I, LXXXIV, Juni 1881. 



