THE SPIRE-BEAltERS. 



235 



THE TRIMERELLID^E. 



The four genera comprised in this family, all apparently restricted to the Silurian seas, occur 

 abundantly in rocks of that age in Sweden, Russia, and Canada. The shell, generally calcareous 

 and often extremely massive, was characterised by a raised and vaulted platform for the attachment 

 of the muscles. The animal was probably Tretenterate, as no support for the brachial organs 

 is preserved. But the occasional presence of rudimentary hinge-teeth and characters of the 

 muscular system indicate that the structure of these extinct forms was somewhat intermediate 

 between that of the Tretenterate and Clistenterate group. 



It includes a number of 



Fig. 10. A, INTERNAL CAST OF DORSAL VALVE OF ORTHIS. 



B, SECTION OF PENTAMERUP, SHOWING CHAMBERS. 



(After Davidion.) 

 A a, Adductor Muscles; v, Veins; o, Ovarian Spaces. 



ORDER CLISTENTERATA. 



FAMILY, THE PRODUCTION.* 



This family ranges in time from the Silurian to the Permian epoch, 

 fossil forms, which vary considerably among 

 themselves. The prominence of the beak and 

 shoulder of the shell suggests the generic 

 name of the type (Fig. 3, B, c). The shell 

 substance was perforated by canals, and the 

 exterior of the ventral valve was often pro- 

 fusely ornamented with tubular spines, some- 

 times restricted to the hinge region. The 

 brachial organs, contained in a depression 

 of the dorsal valve, were not supported by 

 any internal skeleton. In some genera the 

 two valves were united solely by muscles ; in 

 others hinge-teeth and sockets were developed. 

 The distinct impressions of the adductor 

 muscles are very characteristic of this family. 

 Members of the large family of the Strophornenidae appeared in the Cambrian seas, and became 

 extinct in the Upper Liassic period. The valves of these compressed shells were usually concavo- 

 convex, and f iirrowed from hinge to margin. Hinge-teeth were present, and the breathing organs were 

 supported by a short process. The figure of the genus Orthis (10, A), showing the impressions of the 

 mantle, vascular and muscular systems, illustrates the manner in which the anatomy even of fossil 

 species can be determined. In the Pentameridsef (Fig. 10, B), or five-chambered shells, the articulated 

 valves were divided into five parts or chambers. Those at the side were occupied by the arms and 



ovaries, and the V-shaped division near the beak by 

 the digestive organs. The shell is smooth and im- 

 perforate, with a prominent beak. This family was 

 represented only in the Silurian, Devonian, and 

 Carboniferous epochs. 



THE SPIRE-BEARERS. 



The name of this family, which comprises some 



11. A, DORSAL VALVE OF spiRiFEHA GLABRA, of the most beautiful of the numerous fossil forms 

 CARBONIFEROUS. B , DORSAL VALVE OF RHYNCHo- o f the class, is derived from the spiral shape assumed 



NELLA PSITTACEA. (After Davidson and Owen.) , ,. , , . / , 



A- a Bin p A "Y *" fl calcareous labial appendages which nearly till 



' Natural p^Ift ,i n *' Sl I' T ral k''ial Appemlntres. B: rf, Arms in . , , , /T ,. .. , x ,,,, 



vascular system of Mant'e. al Plates; sockets ;, the interior of the dorsal valve (Fig. 11, A). The shells 



are ovate, elongated, and sometimes extended into wing- 

 like expansions. The Spiriferidse appeared in the Silurian seas, and became extinct in the middle of 

 the Liassic period. Among the family of " Little Beaks," or Rhynchonellidse, the valves are strongly 

 articulated, and the generally im perforate shell is three-cornered, with a small-pointed beak 

 (rig. 11, B). The fleshy, spirally-rolled breathing organs are merely supported by a short 

 calcareous process, and are therefore capable of extension beyond the margins of the shell 

 Professor Morse witnessed this operation, and states that the appendages can be unrolled and 



* Latin, producta, produced. t Greek s pente, five ; meros, part. 



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