TURRILITES 59 



G^s— TURRILITES, Lamarck. 1801. 



Animal unknown : shell chambered, turreted, usually sinistral, with numerous angular 

 whorls, twisted spirally round a central cavity, and contiguous throughout on their upper 

 and lower sides : chambers separated by transverse, flexuous septa, deeply sinuated towards 

 the edges, and divided into six branching lobes, which are unsymmetrical, those on the 

 outer side of the whorls being much larger than the corresponding lobes on the inner side ; 

 spire traversed by a continuous tube, situated on the outer side of the whorl, a little below 

 the suture : mouth of the adult shell produced downwards, and bent over somewhat into 

 the form of a hood. 



The genus Turrilites, as it has been usually received, includes, in addition to the shells 

 here classed in it, other chambered, turreted species, with contiguous, rounded whorls, 

 and the siphuncle placed on the middle of the outer side of the whorl. I have ventured 

 to remove these species to the genus Helicoceras, with which they agree in the round form 

 of their whorls, and in the position of their siphuncle, and only differ in having their 

 whorls contiguous instead of separate. The importance of that difference is destroyed by 

 our finding a species with rounded whorls, Helicoceras or Turrilites pohjplocus, Roemer, 

 in which the whorls are separate during the early part of their growth, contiguous in 

 their next stage, and again separate when full grown ; in consequence of which it has been 

 moved from one to the other genus, according to the part of the spire which was described. 

 M. D'Orbigny had already pointed out that there were two very distinct groups of 

 Turrilites, and assigned their characters — 1st, the rotundati, which are those which 1 

 propose to unite to Helicoceras ; 2d, the aiigulati, which form the genus Turrilites, as here 

 retained. 



The produced, arched mouth has not yet been seen in all the Turrilites, but there are 

 indications of it in so many species, that it may be presumed to be universal in the genus. 

 I can find no evidence that this mouth was formed more than once during the life of the 

 animal, as is conjectured by M.. D'Orbigny ('Terr. Cret.,' p. 570); on the contrary, we 

 find that, before the mouth is produced, there is always some modification in the form of 

 the ribs which would betray the previous position of the produced mouth, if such had 

 existed and had been absorbed by the animal ; but no specimens show any such irregu- 

 larity of the ribs in any of the upper whorls, so that we must conclude that the mouth 

 was only produced once, and the different size of individuals of the same species on 

 which this occurs only show that some reached their adult form sooner than others. 



There is very little difference in Turrilites between their forms and ornaments in the 

 young and older stages of their growth, excepting near the produced mouth ; so that the 

 species may be recognised in specimens of any size or age. 



