158 XENOPHORTD^E. 



Genus EUTROCHUS, Whitfield, 1682. 



Conical above, flat or concave beneath, broadly deeply umbili- 

 cated ; aperture very oblique, periphery strongly carinated or 

 expanded ; surface ornamentation unlike above and below. Dif- 

 fers from the umbilicated forms of Trochidae in not forming a 

 columella, and from Risella in being umbilicated. 



E. CONCAVA, Hall. Carboniferous, Illinois, Indiana. 



Genus AUTODETUS, Lindstrom, 1884. 



Shell small, widely conical, sinistral, attached by its truncated 

 apex to foreign bodies, axis im perforate, suture not perceptible 

 externally. A. calyptratus, Schrenek, Silurian of Gotland. An 

 aberrant form placed in this family by Lindstrom. The apical 

 fixation resembles that of certain Vermetidae, but the shell may 

 be a tubiculate Annelid. Anticalyptraea, Quenstedt, 1884, is a 

 synonym. 



Genus CLISOSPIRA, Billings, 1865. 



This curious form from the Silurian of Canada, which I have 

 placed, with some doubt, in Trochiclse, is with equal doubt 

 referred here by Fischer. It is trochiform, with reticulated 

 surface ; there is some evidence that towards the apex it is 

 spirally coiled, but the cavity occupied by the body of the 

 a-nimal appears to be straight and central, with an aperture 

 expanding trumpet-like all around. 



The XenophoricUe have been monographed by Reeve, in Con- 

 chologia Jconica ; by Philippi, in Kuster's Conchylien Cabinet, 

 and more recently by Dr. Fischer, in Kiener's Coquilles vivantes. 

 They first appeared possibly in the Silurian, more probably in 

 the Devonian, and have not been numerous at any period of the 

 geological history. 



