36 INTRODUCTION. 



Pamily.— STROPHOMENIDJE. 



In this family, as in Spiriferidce, there is a want of constancy as regards the shell- 

 canals ; and further investigation is much needed to determine how far the character 

 which is furnished by their presence or absence corresponds with those other characters on 

 which generic distinctions may be most securely based. — The results of the examinations 

 which I have hitherto made are as follows : — 



Orthis. — The prevalent character in this genus is that of perforation, which presents 

 itself in the following species : hiloha, canalis, elegnntula, JlUaria, hi/bridal Michelini, 

 resupinata, striatula, testudinaria. Of nearly all these it is particularly noticeable that 

 the openings of the perforations on the external sm'face of the shell are arranged in regular 

 rows on the ridges of the plications. The only species in which I have not found the 

 perforations to exist, are 0. hiforata and calligramma. 



Orthesina. — So far as I at present know, the shells of this genus are not perforated. 

 The species which I have examined are 0. ascendens, elegans [?], and hemipronites. 



Strophomena. — Here, on the other hand, the perforation of the shells seems to be the 

 rule ; but the perforations are unusually remote from each other (PI, V, fig. 15 a), and the 

 laminae of the shell are bent down round the margin of each, so as to give it a somewhat 

 infundibular character in vertical or oblique sections (fig. 14 h b, fig. 15 3 (5). The shells 

 of this genus which have fallen under my examination, have for the most part undergone 

 so complete a metamorphosis, that their original texture is almost entirely obscured ; and 

 even the perforations, when filled-up by a matrix of nearly the same appearance with the 

 metamorphic shell, may pass undetected, save when carefully looked for : I have found 

 their existence to be sometimes more certainly revealed by vertical than it is by horizontal 

 sections. The species which I have examined are alternata, depressa, funiculata, and 

 planoconvexa ; in all of which the perforations are clearly distinguishable. 



Lept^na. — In one of the most characteristic species of this genus, Z. transversalis, 

 the shell is obviously traversed by perforations, resembling those of Strophomena depressa in 

 their size, distribution, and arrangement. I have found similar perforations in L.semiovalis. 

 On the other hand, I have examined specimens of L. Davidsoni and L. oblonga without 

 being able to recognise them ; in these specimens, however, the shell had undergone such 

 a complete metamorphic action, that it would be very unsafe to affirm the non-existence of 

 perforations in these two species. 



Family.— PRODUCTID^. 



In all the genera of this family large perforations exist, resembling those of 

 Strophomena depressa in their general aspect, and in the infundibular arrangement of the 



