SPECIES OE MTTPvCHISONlA. 



619 



on the whorls by the strong arching of the lines of growth, both in 

 Turritella and Murchisonia. But there is this difference between 

 Turritella and Murchisonia, that, whereas the successive filling- 

 up of the sinus in Murchisonia forms a distinct band, which is 

 limited on each side by keels or by grooves, it never seems to 

 form a distinct band on all the whorls of Turritella. In T. run- 

 cinata and T. accisa, however, the sinus lies between two carina© ; 

 and, on the other hand, G. Lindstrom says that in Murchisonia 

 attenuata, His., " near the aperture in large specimens, the trans- 

 verse ornamental striae above and beneath are confluent with the 

 band, without any separating or bordering lines, and it continues 

 only as an elevated ridge, sometimes not clearly distinct from the 

 surface " (' Silurian Gasteropoda and Pteropoda of Gotland,' p. 130). 

 The mouth of Murchisonia is different in form from that of Tur- 

 ritella, being longer and slightly channelled below, while that of 

 Turritella is, as a rule, rounded or subquadrangular, though some 

 species, such as T. admirahilis, Watson, and T. lamellosa, Watson, 

 are slightly channelled at the base of the pillar. In the group 

 Hormotoma, Salter says that the mouth is rounded ; but I have not 

 seen any sufficiently well-preserved specimens to show this ; the 

 specimen from which his type is taken is evidently broken at the 

 base of the mouth. 



Mr. Boog Watson also points out two remarkable features in 

 Turritella, viz., the possession of a system of microscopic spirals, 

 which covers the entire shell, and also the existence of an epidermis. 

 If this latter had existed in Murchisonia, we could hardly expect 

 any trace of it to be preserved in shells of so great an age, unless 

 under exceptional circumstances ; and I have not come across any 

 species of Murchisonia sufficiently well preserved to show whether 

 they possessed the microscopic spirals or not. Another point to be 

 noticed is that Turritella possesses an operculum, and I am not 

 aware of any opercula being found which can with certainty be 

 referred to Murchisonia. There is also a greater variation in the 

 spiral angle in Murchisonia than in Turritella, some of the shells 

 being almost buccinoid in form. 



No observations seem to have been made upon the animals of the 

 shells with a deep sinus in the outer lip, and it would be interesting 

 to know if there is any difference between their form and that of 

 the animals belonging to the shells which have the outer lip merely 

 sinuated. Thus some idea might be formed of the relative value of 

 the sinus, with regard to making generic distinctions. 



The researches of Dall upon the animals of the recent species of 

 Pleurotomaria have led to their being separated from the Haliotidse, 

 and being assigned to a distinct place in a family of their own. G. 

 Lindstrom- refers the genus Murchisonia to the family Pleuroto- 

 mariadae, which position it may be well for it to retain, until further 

 research throws more light upon its affinities. 



At present the synonymy of this genus stands as follows : — 



Muricites (pars), E. F. von Schlotheim, 1820, ' Petrefactenkunde/ 

 p. 145. 



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