266 



NATURAL HISTORY. 



protruded nearly twice the length of the shell. They often remained extended a. short distance 

 for hours. Their movements were very sluggish, but the cirri were constantly in motion, and the 

 shells sometimes closed upon the arms before they were retracted. This family was represented in 

 Silurian times, by the type and other forms, in some of which the spires were coiled round and round 

 like a watch spring. The Rhynchonellidse culminated in the Jurassic seas ; six species only survive. 

 Of the allied genus Atretia, one Atlantic species is known, and a second was recently dredged by Mr. 

 John Brazier in Australian waters. 



THE TEREBRATULIDJE.* 



This, a most extensive family, comprises a great number of generic and sub-generic forms, which 

 are usually characterised by the large-sized perforation in the ventral or pedicle valve, for the passage of 

 the peduncular organ of attachment. The calcified support for the respiratory and alimentary organs 



exhibits considerable modifications, which serve to 

 distinguish one genus from another. Sometimes 

 the loop is short and simple (Fig. 12, A) ; in the 

 form of a ring (B) ; or it is long and doubled back 

 on itself, as in Fig. 5, B, I; attached twice (Fig. 

 12, c) or three times (E) to the ridge or septum 

 running down the centre of the valve. Again, the 

 respiratory and food-securing organs may be sup- 

 ported by an anchor-shaped process (D), or by 

 short ridges developed along the margins of the 

 valves (G). It seems probable that the ringed 

 type of loop is a further modification of the 

 simplest and highest type that of Terebratula. 

 For the embryological researches of Friele have 

 revealed the fact that in other genera the loop 

 passes from a complicated to a simple type, as- 

 suming at successive stages shapes which are 

 absolutely identical with the characters of those of full-grown individuals of other genera. This dis- 

 covery, however suggestive as throwing light on the genetic descent and relationship of the Brachio- 

 poda, does not affect the value of the calcified loop for classificatory purposes, as each genus is 

 distinctly characterised by its special form of loop in adult animals. The Terebratulidae include the 

 majority of existing species of the class. It was represented in the Silurian seas, and steadily 

 increased in numbers until the dawn of the Tertiary epoch, since when its specific representatives 

 have been somewhat reduced. 



THE THECIDID^E. 



The genus Thecidium is the only member of a family which apparently came on the scene of life 

 in the Triassic age. Living species attached by their ventral valve to corals, moss-animals, and 

 dead shells of their race, abound in the Mediterranean sea, where they were dredged 

 in great numbers by M. Lacaze-Duthiers, who kept them alive for several weeks, and 

 gives the following account of their habits. The animals are certainly sensible to light, 

 for the valves were abruptly closed on the interposition of any shadow. They were 

 generally opened very slightly ; the upper moved at right angles on the hinge line, 

 and shut with the rapidity of lightning when the animal was disturbed. The ciliated 

 folded breathing organs, supported by a calcareous loop, are sunk in hollows in the 

 attached valve. The shell is small, thick, strongly articulated, and its substance per- 

 forated by canals. 



The Brachiopoda apparently prefer rocky shores to muddy bottoms ; they are much 

 localised and generally occur in great abundance in their favourite haunts. Out of the one hundred 

 and thirty known genera, only twenty-two are represented by living forms. Many of these survivors 

 have a wide geographical range, and occur at most varied degrees of depth. Ten species are 

 found off the British coasts. The Lingulidse, as a family, exist in very shallow water living half- 



* Latin, terclratus, perforated. 



Pig. 12. A, LOOP OF 



C, TEHEBRATELLA ; D, 



AHGIOPE. (After Davidson.) 



LIOTHYRIS J B, TEKEBRATl'LIXA J 

 BOUCHARDIA ; E, MEGERLIA ; G, 



h, Hinge ; I, Loop ; g, Septum. 



Fig. 13. - 



THECIDIUM 

 M E I) I TEH- 

 RAN BUM, 

 AS IX LIFE. 

 (After Lacaze- 

 Duthiers.) 



