Animal Tethys. 



Testa bivalvis, labiis margine antico incumbentibus . 



Cardo dentibus 3: omnibus approximatis : lateralibus apice divergentibus. 

 Vulva & Anus Distincta. - /Linnaeus, 115%// 



Shell large, inequilateral and thick; lunule is large, cordate and bounded 

 by an incised line; escutcheon defined by a high ridge which varies in 

 sharpness; inner margin crenulate; pallial sinus medium in size, pointed; 

 three cardinals in each valve; anterior cardinals of both valves simple; 

 left, middle cardinal bifid; left, anterior cardinal high and pointed, right 

 middle and posterior cardinal bifid; left, posterior cardinal very thin and 

 hardly differentiated from the nymph plate; no laterals in either valve; the 

 nymphs of both valves very coarsely rugose; exterior usually smooth or with 

 concentric ribs or striations. Gives also complete synonymies for V. 

 meraenaria and V. oampeaniensis . Describes also 78 other species and sub- 

 species of Venus. - J.L.M. 



1431 



Pannella, Giorgio. 1972. 



Palaeontological evidence on the Earth's rotational history since early 

 Precambrian. Astrophys. Space Sci . 16(2): 212-237. 



Daily growth layers in calcified structures of modern and fossil organisms, 

 especially bivalve mollusks, are arranged in seasonal and tidal patterns and 

 provide evidence on length of lunar month and year in the geological past. 

 Growth patterns in shells of Meraenaria meraenaria are clearly developed and 

 are among the most complex, so that they provide maximum information. Data 

 from Miocene M. oampeaniensis oohlockoneensis and Recent M. meraenaria and 

 M. oampeaniensis and other calcareous remains demonstrate that the number of 

 days/lunar month and days/yr has decreased significantly since Ordovician 

 time, but not necessarily at a uniform rate. - J.L.M. 



1432 



Pannella, Giorgio. 1975. 



Palaeontological clocks and the history of the earth's rotation. In Growth 

 Rhythms and the History of the Earth's Rotation. G. D. Rosenberg and S. K. 

 Runcorn (eds.). John Wiley & Sons, London: 253-284. 



J. W. Wells was the first to propose use of fossils as "geochronometers" . In 

 intertidal Mercenaria mercenaria , organic lines are formed during periods of 

 shell closing at low tide. The periodicity of line formation is lunar, 12 hr 

 25 min in semi-daily tides or 24 hr 50 min in daily tides. Because the lunar- 

 day rhythm is bimodal, a lunar increment may consist of 2 growth layers and 

 2 organic lines, when 2 low tides interfere with internal calcification 

 rhythms. The growth layer is deposited rapidly in 2 to 3 hrs during rising 

 tides, as calcification is accelerated after a period of enforced retardation. 

 Most rapid deposition was observed during evening rising tides. The 

 propitious time for calcification each 24-hr period is dictated by internal 

 circadian rhythms and external factors such as light and temp. Intertidal 

 growth patterns are not caused entirely by tidal rhythms, but by interplay 

 of solar, circadian, and lunar rhythms. Growth patterns of M. meraenaria 

 in New England are different from those in Florida because these conditions 

 are different. Animals in intertidal areas show a "switch zone" in shell 

 structure, which is caused by the tidal phase 50-min shift with respect to 

 sidereal day. The shift compresses the time of calcium carbonate deposition 

 in one direction until it "snaps back", missing a day every fortnight, 

 consisting of 13 1/2 lunar days (14.8 sidereal days). Different types of 

 switch zone have in common the fact that one growth line used as a reference 

 to measure the "daily" spacing suddenly repeats at half the previous or 

 subsequent distance. When this happens the sequence of symmetrical increments 

 changes to sequences of asymmetrical increments. In earlier studies, e.g., 

 Pannella and MacClintock (1968) , the coincidence between number of counted 

 bands and number of days was fortuitous, caused by the particular natural 

 conditions prevailing at the time of the experiment. For some purposes the 

 loose definition that increments are daily is acceptable, but for precise 

 geophysical interpretation a strict separation of solar and lunar increments 

 is necessary. Conclusions from earlier work must be reassessed. - J.L.M. 



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