Miscellaneous. 109 



On a new Lamellibranch (Scioberetia australis) commensal with an 

 Echinoderm. By F£lix. Bernard. 



In studying the collection of Echinoderms made by the Cape- 

 Horn Expedition in the years 1882-1883 I have had occasion to 

 examine several specimens of a Spatangid, Tripylus excavatus, Phil., 

 and I discovered that this sea-urchin is viviparous. But, while the 

 majority of individuals carry young in process of development in 

 the depressed ambulacral zones, in two specimens the young were 

 absent and were replaced by a Lamellibranch, the maximum size of 

 which does not exceed 3 millimetres. I have been able to study 

 this animal by the dissection of three examples, and by means of 

 sections in the case of a fourth : the state of preservation of the 

 specimens is highly satisfactory. 



The shell is entirely contained in the thickness of the mantle ; it 

 does not cover half the breadth of the animal. The mantle, on the 

 contrary, envelops it completely. It is prolonged in front by a 

 groove, which recalls the siphon of Gastropods ; it is united together 

 in the median line behind, and leaves only a posterior orifice, in the 

 centre of a circular area. The shell, which is very thin, equivalve, 

 and posteriorly truncate, bears radiating striae, and its ventral 

 margin is slightly plicate. No muscular nor pallial impressions. 

 In the umbonal region, which is not projecting, the embryonic 

 shell is seen. The cardinal mai'gins of the two valves are in contact 

 only along the embryonic shell ; further on they are separated and 

 connected by a delicate epidermic ligament. The ligament proper 

 is internal and occupies the whole of the embryonic hinge ; on 

 dissolving it by means of hypochlorite of soda, it is seen that the 

 hinge has retained exactly the conformation that it possessed in the 

 embryos, which are met with in abundance with the adults. It 

 exhibits at its summit a little ligamentary pit interrupting the 

 cardinal margin, and on each side parallel crenatures similar to 

 those which exist in the embryos of a multitude of Acephala 

 (Nucula, Mytilus, Area, &c). In the adult the ligament runs out 

 on to all these structures, and neither cardinal plateau nor teeth, 

 properly so-called, are formed. 



As opposed to this persistence of the embryonic type of shell the 

 internal organization exhibits an advanced specialization. 



The adductor muscles, greatly reduced in size, are seen (in a 

 section) in their normal position. The foot is but little developed 

 and is divided into an anterior and a posterior lobe ; in the retracted 

 condition due to immersion in spirit it displays a deep longitudinal 

 groove, which manifestly corresponds to a creeping sole. No byssus. 

 Nervous system normal, with very large ganglia. 



Alimentary canal very simple. The mouth opens on the dorsal 

 face of the visceral mass ; on each side it is joined by a deep groove, 

 bounded by two lips which represent the rudimentary labial palps, 

 and extend on the sides as far as the base of the gills. (Esophagus 

 straight, dorsal ; stomach spacious, with a broad opening from the 

 digestive gland (liver), which occupies the two anterior thirds of the 



