102 SUPPLEMENT TO THE 



incurved and truncated by a minute oval foramen ; surface smooth, marked by a few 

 concentric lines of growth. In the interior of dorsal valve the spirals are composed 

 each of not more than six or seven convolutions. 



Length 6, breadth 5 lines, but the generality of specimens are much smaller. 



Obs. — The small dimensions attained by this species caused the Rev. Norman Glass 

 much trouble in his patient endeavours to work out its interior arrangements. A great 

 number of English specimens were operated upon, as well as a number of Swedish 

 ones kindly contributed by Professor Lindstrom, of Stockholm. Mr. Glass has shown in 

 a very plain manner the accessory lamellae, also the hook-shaped attachments to the 

 hinge-plate ; but, on account of the smallness of the specimens, it was not possible to 

 expose the loop or its attachments to the primary lamellse in the same manner he had so 

 successfully achieved in Athyris piano- sulcata. We are both convinced that Sowerby's 

 species is an Athyris. It is probable also that some other of our British Silurian spiral- 

 bearing species may be referable to Athyris, only the material for proving this has not 

 been hitherto obtained. 



I was completely mistaken when, at p. 115 of my 'Silurian Monograph,' I considered 

 Sowerby's T. lavluscula to be an immature form of Hall's Meristina nitida. Since then, 

 thanks to Mr. Maw's extensive washings of Lower- Wenlock Shales, as well as hand 

 pickings from the detritus of the Wenlock Limestone in the old quarries of Benthall Edge, 

 I have been able to examine a large number of specimens of Sowerby's species. I have 

 also, thanks to Professor Hall and Mr. Whitfield, been able to study a number of specimens 

 of the so-called Meristina nitida from the Niagara group, Waldron, Indiana ; and I have 

 come to the conclusion that Meristina tiitida is a synonym of Meristina didyma, and that 

 Athyris laviuscula is both generically and specifically distinct from Caiman's species as 

 well as from Professor Hall's. The searching examination of the interior of both 

 these species by the Rev. Norman Glass has left not a shadow of doubt upon this 

 subject. 



In a specimen one line and a half in length, cleared out by Mr. Glass, there were 

 only three coils in each spiral. In another, four lines in length, four coUs ; and none 

 of the largest specimens have shown more than six or seven in each spiral. Athyris 

 laviuscula varies a good deal in shape. Some young specimens are as wide as long, but 

 full-grown specimens are elongated, oval, truncated in front, and tapering at the beak. 

 Large examples, measuring six lines in length, seem extremely rare ; most of the specimens 

 average from three to four lines in length. A few examples were also obtained by Mr. 

 Maw from the washings of some three tons weight of Lower-Ludlow Shales from the 

 railway-cutting between Wenlock and Presthope in Shropshire. It appears likewise 

 not to be very rare in the Wenlock scries of the Island of Gothland in the Baltic. 



