LECTURES ON MOLLUSCA. 101 



in very thick shelly tubes, often a yard long and two inches across, of 

 prismatic structure like the Pinnas and Belemnltes. At the outer 

 end, the pipe is divided across for a considerable distance. It is said 

 that these Septarias have no hivalve shells at all ; but that the foot-end 

 is closed in by a cleft shelly plate. 



The Ship-worms are connected with the ordinary borers of the fossil 

 genus Teredina; in which the animal is as short as a stretched-out 

 Pholas, enclosed in a thick tube, somewhat divided at the outer end. 

 The valves, which were probably free in the young state, are soldered 

 into the tube in the adult, so that the animal was completely encased. 

 Fossil Ship-worms are found in fossil wood as far back as the Lias. 



Family GASTROCH^NIDJE. (Tube-Shells.) 



The valves of Gastrochcena have a true ligament, and move freely in 

 their burrow, so that the little finger-like foot which protrudes from 

 the otherwise closed mantle, is able to perform as much abrasion as the 

 stout organ of the Pholads. When the Gastrochcena does not burrow 

 in solid stone or shell, it forms an irregular club-shaped tube, in 

 which it encloses both its pipes and its valves. In Chcena, which 

 burrows in sand, the tube is straight ; and the part which contains 

 the rectangular valves is partitioned off from the pipe portion. The 

 very curious shells of Bryopa are like a Teredina with one valve loose, 

 and the other cemented into the tube. The animal is stumpy and 

 irregular, with rather short fringed pipes, and has the general aspect 

 of a tunicary in a shelly case. It is difficult to understand the use of 

 the single loose, and the single fixed valve: Dr. Darwin might regard 

 it as a Gastrochcena passing into a Teredina, or vice versa. The fossil 

 genus Clavagella differs only in having the closed pedal end sur- 

 rounded with a bunch of short tubes, in which respect it forms an 

 interesting passage to the Watering-pots or Aspergillum group. 



At first sight a " Watering-pot shell" would not be supposed to 

 have any connection with ordinary bivalves. It consists of a tube, 

 open at one end, at the other closed by a disk, full of holes, and gen- 

 erally surrounded by frills of shelly tubes. On looking attentively 

 near the rose however, we shall see two irregularly imbedded valves, 

 which are small in Aspergillum (the principal part being free inside) 

 and large in Penicillus, and which show the intimate relation of the 

 creature to Clavagella, Ghana, &c. In the middle of the rose is often 

 a slight chink for the rudimentary foot. The open end, which ap- 

 pears above the sand, and is often adorned with one or more ruffles, 

 affords an orifice for the breathing pipes. In Foegia the valves can 

 scarcely be seen outside. The animal of Humplireyia attaches itself 

 when young by the front edges of the valves, which it gradually ex- 

 tends into a tube. 



Family SAXICAVIDJE. 



The Sax cava group are like shortened Gastrocligenicls, without any 

 shelly tube. They sometimes bore, but more often nestle in holes made 

 by other creatures, or in corners of rocks and roots, mooring themselves 



