264 



NATURAL HISTORY. 



calcareous labial appendages (Figs. 5, B, I) are therefore one of the most important features affecting 

 the life and organisation of the animal inhabiting the shell, and their value as classificatory agents is 

 apparent. Frequently preserved in a fossil state, there can be no doubt that they afford the simplest 

 and readiest mode of determining the true relationship of one adult form of Brachiopod to another, 

 and one that is infinitely preferable to the purely external characters of these very variable shells. In 

 the subjoined table of families, typical genera are placed first, and those represented solely by extinct 

 forms are printed in italics : 



Genera. 

 Lingula, Glottidia, Lingulops, Obolus. 



Family. 



LlNGULIDJE 



OEDER TEETENTERATA 



CLISTENTERATA 



CRANIAD-E 

 Trimerellid<e 



( Productida 

 Strophomenidte . 

 Pentameridce 



Spiriferida 



RHYXCHONELLIDJE 



TEREBRATULIDjE 



THECIDIDJE 



Discina, Discinisca, Trematis, Siphonotrcta. 

 Crania, Craniops, Craniscus. 

 Trimerella, Monomorella, Dinobolus. 



Productus, Chonetes, Strophalosia. 

 Strophomena, Orthis, Leptcena. 

 Pentamerus, Strieklandina. 

 Spirifer, Spiriferina, Meristella, Athyris. 

 Rhynchonella, Rhynchopora, Atrypa. 

 Terebratula, "Waldheimia, Argiope, Laqueus, 



Kingena. 

 Thecidiiun. 



ORDER TRETENTERATA. 



FAMILY, THE LINGULID^.* 



The typical genus Lingula, one of the most ancient and persistent of Brachiopoda, is, as might be 

 expected, a very hardy animal. The hingeless valves of the usually green or dusky brown shells are 

 nearly equal in size, and it is from their elongated or tongue-like shape that the family name is 

 derived (Fig. 7). The thick fleshy breathing organs with inwardly-directed spires are unsupported by 

 any elaborate calcified process, but merely strengthened by a simple longitudinal plate or septum, rising 

 from the centre of the dorsal valve. Professor Morse describes the American and Japanese species as 

 living free and partially buried in sand, and further states that " the anterior borders of the mantle 

 contract in such a way as to leave three large oval openings, one in the centre and one on each side. 

 The bristles, which are quite long in this region of the animal, so arrange themselves as to continue 

 these openings into funnels, and entangle the mucus escaping from the animal. A continual 

 current is seen passing down the side funnel and escaping by the central one. " They bury themselves 

 quickly in the sand, and the lengthy and worm-like peduncle agglutinates a sand-tube. 



THE DISCINID^E. 



This venerable family is represented by the long-lived genus Discina in the existing oceans. 

 The two horny unarticulated valves are circular or disc-like, hence the name. The upper, resembling 

 that of a limpet, is smooth with an almost central apex. The lower is perforated for the passage of 



the short plug, or byssus, by means of which they adhere to each other. 

 No external skeleton is developed for the support of the breathing 

 organs, which curve backwards, and end in small spires directed down- 

 wards. Very little is known of the habits of the living species which, 

 widely distributed, exist both in shallow water and at very considerable 



depths. 



THE HELMET-SHELLS. 



The Craniadsef attach themselves by the flat or under valve to 

 other bodies. The helmet-shaped appearance of the upper valve gives 

 the name to the group. On the interior of the thick and hingeless 

 calcareous shell four distinct muscular impressions are visible. The 

 arms (Fig. 9, b) are free, coiled in spirals towards the concave space of the 

 dorsal valve, which is generally firmly attached to foreign objects. The two valves are only united by 

 muscles, and separate readily on the death of the animal, hence the large number of single valves 

 occurring in a fossil state. They range in time upwards from the Silurian epoch. 

 * Latin, lingula, a little tongue T Greek, kranos,*. helmet. 



Fig. 9. CRANIA ANOMALA. 



(After Davidson.) 



b, Brachial Organs ; c, Two Muscular Im- 

 pressions. 



