CARBONIFEROUS FOSSILS OF IRELAND. 27 



GASTEROPODA. 



The modern tribe Pec^iHi'i7-fl?ic/i/a?« includes all those molluscs whose body is covered by a spiral univalve 

 shell ; they have the head distinctly marked and pro^aded with eyes and tentacula, and the under surface of 

 the body formed into a disc-like foot for walking. The tribe is divisible into the following groups : 



First, the Zoophaga of Lamarck, or the carnivorous shell-fish: these are among the most highly organized 

 of the Gasteropoda; they have a distinct head, furnished with two tentacula, and perfectly formed eyes. 

 Their shells are distinguished from the other univalves by having a deep notch or canal at the base of the 

 aperture, which serves as a protection to the respiratory siphon of those animals. Of this group there 

 are very few examples to be found in the mountain limestone ; and these, I think, chiefly belong to the 

 family FusincB ; the numerous shells, called Buccinum by geologists, not belonging to that group, inasmuch 

 as their base is never truncated ; they would rather approximate to the genus Chrysodomus or Macrocheilus, 

 in wliich the general form is buccinoid, and the basal channel is always shorter than the spire. To this 

 group also belong those shells formerly described as Melania (but now separated from that fresh- water genus 

 by Professor Phillips, under the generic name of Loxonema) ; they are analogous in the present group to 

 the TurritellcB among the Phytophaya. 



The next group is the Phytophaya. These are less constant in their characters than the last, but the 

 greater number live upon vegetables, and have the aperture of the shell entire, or wanting the notch of the car- 

 nivorous group. 



The Phytophaya, or plant-eating Gasternpods, include nearly all the univalve shells of the Palaeozoic rocks ; 

 all those belonging to the genera Pleurotomaria, Turbo, Turritella, and Euomphalus, being clearly of this 

 tribe ; a satisfactory proof of the abundance of fuci or sea- weeds at these periods, although their remains are not 

 found in the fossil state. As the animals of this division have no respiratory siphon, the shells are easily dis- 

 tinguished from those of the carnivorous Gasteropods by the latter having the base deeply notched to allow the 

 exit of the respiratory tube, while those of the present tribe, being destitute of that organ, have the aperture 

 round and entire ; they are all spiral, turbinated shells, wliich at once distinguishes them from the Scutibran- 

 chia, or the limpets {Cyclobranchia) with which many authors confound them. One anatomical fact of impor- 

 tance to the fossihst seems to have been entirely overlooked, namely, that the sexes are distinct and in difle- 

 rent individuals, the female shells having the body whorl invariably more ventricose than in the male specimens, 

 so that if unacquainted with the structure of the animals, one might be led to describe the sexes as distinct 

 species. The majority of the mountain limestone shells belonging to this group appear to enter into the family 

 Turbidee, or that family which connects the Phytophagous with the Zoophagous Gasteropods, these animals 

 exhibiting much of the structure of the carnivorous type, while their shells are strictly those of the Phytophaya, 

 another instance of the importance of attending to the anatomy of the creatures, as the mere conchologist might 

 lead to error, by denying the existence of carnivorous shells, at periods when they might have been abundant, 

 the popular idea being that every carnivorous shell must have a notch for the respii'atory tube ; it is, however, 

 well known that a number of recent shells of this group have the base of the shell entire, although the animal is 

 carnivorous. 



After the Pectinibranchiata we have the Scutibranchia, which are known at once from the last by their 

 funnel-shaped shells, which are scarcely ever spiral. 



The next group is the Cyclobranchia, or limpets and chitons, the first having a simple cup-shaped shell, 

 and the other one formed of eight transverse, imbricating, shelly plates, scarcely ever found in the fossil 

 state. 



Next we have the Tectibranchia, or Bulks, whose mantle forms two fin-shaped lobes, by which they can 

 swim with facilitv : they form the passage to the Cephalopoda. 



The remaining groups are not interesting to the Paleontologist. 



